Outlines of Zooloc^v. T- Arthur Thomson, 8th edition. 28 + 972 pj). '528 ligs. Oxford Uni- versity Press. 1929. This admirable survey of the animal kinodom from Amoeha to man is remarkable for the wealth of information systematically arranged in a volume of handy size. It contains essentiallv ^ - tlie information that the l)ej^innin2: zoologist w should have, and the skillful use of three points £ ^ of type gives a sense of perspective that is im- ■p portant, especiall\- in a work of this kind. In the ^ 03 ])resent edition the author has had the help of ^ his son, Dr. D. L. Thomson, in addni.e^ more ^'^' ])hysiolocrical material, and of J\Ir. R. M. Neill on H<; the structure and development of the mud-fishes. Hie first six chapters deal in a general wav w"th Q>w physiology, morphology, palaeontology, the doc- "^^ trine of descent, etc. Then follows an account P o . ' (DO of each of the principal phyla, including general *^^ characters, followed by descriptions of typical (D forms of special interest, then classification, struc- ^ ture, life-history, ecology, and other topics, suchj as parasitism and relation to disease. The illus- trations are clear and significant, and for the most part are original. The final chapters deal with geographical distribution and the factors in or- ganic evolution ; then follow test questions, an ex- cellznt list of books of reference, and an index. — R. P. BiGELOW. ^'^ ■\ Reviewed in the "Collecting Net' August 22, 1931, by Dr. R, P. Bigelow, and presented to the Liorary of the Marine Biological Laboratory OUTLINES OF ZOOLOGY / / - OUTLINES OF ZOOLOGY BY J. ARTHUR THOMSON, M.A, LL.D. REGIUS PROFESSOR OF NATURAL HISTORY IN THE UNIVERSITY OF ABERDEEN ; AUTHOR OF " THE STUDY OF ANIMAL LIFE," " THE SCIENCE OF LIFE, «'THE BIOLOGY OF THE SEASONS," "HEREDITY," "DARWINISM AND HUMAN LIFE," " THE SYSTEM OF ANIMATE NATURE," " THE NEW NATURAL HISTORY " EIGHTH EDITION, REVISED, WITH 528 ILLUSTRATIONS HUMPHREY MILFORD OXFORD UNIVERSITY PRESS London Edinburgh Glasgow Leipzig New York Toronto Melbourne Cape Town Bombay Calcutta Madras Shanghai 1929 First Edition i8q2 Second Edition i8q§ Third Edition . . iSgg Fourth Edition Decetnber IQ07 Fi/th Edition November igro Sixth Edition April igi4 „ ,, Second Impression . . December jqi6 „ „ Third „ . . January iQig ,, ,, Fourth ,, . . October iqiq „ ,, Fi/th „ . . September IQ20 Seventh Edition March ig2i Eighth Edition October igzg PRINTED IN GREAT BRITAIN BY MORRISON AND GIBE LTD., LONDON AND EDINBURGH \ PREFACE TO THE EIGHTH EDITION In this new edition more account has been taken of the physiological aspect, and in this connection I have been helped by my son, Dr. David Landsborough Thomson. More space has also been devoted to the structure and development of certain types, such as the Mud-fishes, which were too briefly discussed in previous editions ; and here I have been indebted to the assistance generously given by Mr. R. M. Neill, M.C., M.A., Lecturer on Zoology in this University. I have also to thank Prof. W. Rae Sherriffs, D.Sc, for many useful suggestions. Numerous figures have been added, mostly drawn from specimens. J. A. T. The University, Aberdeen, August 1929. PREFACE TO THE SEVENTH EDITION This book is intended to serve as a Manual which students of Zoology may use in the lecture room, museum, and laboratory, and as an accompaniment to several well- known works, cited in the Appendix, most of which follow other modes of treatment. To numerous authorities I acknowledge an obvious indebtedness, a detailed recognition of which would be out of place in a book of this kind. I must, however, acknowledge that in the preparation of a previous edition I had throughout the able assistance of Miss Marion Newbigin, D.Sc, and I have also been aided by sugges- tions from various kindly critics, especially Professor W. C. M'Intosh, Professor P. J. White, the late Dr. Ramsay Traquair, Dr. John Beard, the late Mr. J. G. Goodchild, Dr. Arthur Mastermait, Dr. John Rennie, Dr. W. D. Henderson, Mr. E. S. Russell, Mr. W. P. Pycraft, Mr. C. Tate Regan, and Professor H. J. Fleure. For most of the figures, I am indebted to my artist friends, Mr. William Smith, Miss Florence Newbigin, vu Vlll PREFACE TO THE SEVENTH EDITION Miss E. M. Shinnie, Miss Alice M. Davidson, and the late Mr. George Davidson. In almost every case the illustra- tions have been derived from original memoirs and works of reference, or drawn from specimens. J. A. T. The University, Aberdeen, October 1920. CONTENTS GENERAL CHAPTER I PAGE General Survey of the Animal Kingdom . . i CHAPTER H Physiology ....... 20 CHAPTER HI Morphology ....... 4° CHAPTER IV Embryology ....... 60 CHAPTER V^ Palaeontology ....... 98 CHAPTER VI Doctrine of Descent . . . . .105 iz 37951 X CONTENTS INVERTEBRATES CHAPTER VII PAGE Protozoa. ....... 109 CHAPTER VIII Sponges . . . . . . . -153 CHAPTER IX CCELENTERA . . . . . . . 167 CHAPTER X Unsegmented Worms . . . . . .210 CHAPTER XI Annelids ....... 244 CHAPTER XII ECHINODERMS ....... 289 CHAPTER XIII Arthropoda ....... 319 CHAPTER XIV Onychophora or Prototracheata, Myriopoda, and Insecta ....... 361 CHAPTER XV Arachnoidea and Pal^ostraca . . . .412 CHAPTER XVI Molluscs ....... 433 CONTENTS XI VERTEBRATES CHAPTER XVII Hemichorda • • PAGE . 489 CHAPTER XVIII Urochorda • • f • 499 CHAPTER XIX Cephalochorda • * • t • . 515 CHAPTER XX Structure and Development of Vertebrates . .529 Cyclostomata CHAPTER XXI . 574 Fishes CHAPTER XXII • • . 587 Amphibia CHAPTER XXIII 649 Reptiles CHAPTER XXIV . 683 Birds CHAPTER XXV 725 Mammals CHAPTER XXVI . 776 Xll CONTENTS GENERAL CHAPTER XXVII Distribution . . . . ' . . . 895 PAGE CHAPTER XXVIII Theory of Evolution. ..... 908 TEST QUESTIONS FOR STUDENTS . . .915 APPENDIX ON BOOKS . . . . .931 INDEX 937 LIST OF ILLUSTRATIONS -♦ after Huxley FiG. 1. Duckmole {Ornithorhynchus) ., . . • 2. Phenacodus, a primitive extinct Mammal — after Cope 3. Extinct moa and modern kiwi — after Carus Sterne 4. Crocodiles ..... 5. Salamander, an Amphibian 6. Queensland Dipnoan (Ceratodus) 7. Lancelet, Amphioxus — after Haeckel 8. Ascidian or sea-squirt — after Haeckel . 9. Cephalopod (paper nautilus, female) 10. Fresh-water crayfish {Astacus), a Crustacean 11. a. Caterpillar ; b, pupa ; c, butterfly 12. Spider ..... 13. Crinoid or feather-star 14. Earthworm .... 15. Bladderworm stage of a Cestode — after Leuckart 16. Sea-anemones on back of hermit-crab — after Andres . 17. Fossil Foraminifera (Nummulites) in limestone — after Zittel 18. Diagrammatic expression of classification in a genealogical tree ...... 19. Diagram of Vertebrates .... 20. Diagram of Invertebrates. 21. Diagrams of reflex actions — modified from Bayliss 22. Three ciliated cells. .... 23. A smooth or unstriped muscle-cell, slowly contracting 24. A piece of striped muscle fibre with its nerve-endings 25. Diagram of cell structure — after Wilson 26. Structure of the cell — after Carnoy . " 27. Fertilised ovum of Ascaris — after Boveri 28. Diagram of cell division — after Boveri . 29. Karyokinesis — after Flemming . 30. Diagrammatic expression of alternation of generations 31. Diagram of ovum., showing diffuse yolk granules xiii PA.GE 2 3 3 4 5 5 6 7 9 9 9 10 10 II II 12 13 15 18 19 27 48 49 50 52 54 55 55 57 65 67 XIV LIST OF ILLUSTRATIONS FIG. 32. Diagram of a typical spermatozoon 33. Forms of spermatozoa (not drawn to scale) 34. Diagram of maturation and fertilisation. (From " Evolution of Sex ") . 35. Oogenesis and spermatogenesis — after Boveri . 36. Fertilisation of egg-cell — after Fol 37. Diagram showing relative size of an egg-cell and a sperm-cell 38. Fertilisation in Ascaris megalocephala — after Boveri . 39. Modes of Segmentation ..... 40. Life-history of a coral, Monoxenia darwinii — from Haeckel 41. Embryos — (i) of bird ; (2) of man — after His. The latter about twenty-seven days old 42. Larvae of common eel ..... 43. Mendelian inheritance illustrated in wood snail {Helix nemoralis) ...... 44. Gradual transitions between Paludina neumayri (a), the oldest form, and Paludina hcernesi (/) — from Neumayr 45. Structure of Amoeba proteus — after Lucy A. Carter 46. Life-history of ^wcB&fl ..... 47. Actinophrys sol (Sun-animalcule) — after Grenacher 48. Polystomella, megalospheric form, with large central chamber (M.) and one nucleus (N.) — after Lister . 49. Polystomella, microspheric form, with small central chamber (c.c.)> numerous nuclei (A^.), bridges of protoplasm between chambers {B.) — after Lister 50. Paramcecium in longitudinal optical section, and dividing — after BUtschli ..... 51. Conjugation of Paramcecium aurelia — four stages — after Maupas ...... 52. Diagrammatic expression of process of conjugation in Para moecium aurelia — after Maupas 53. Vorticella — after Biitschli .... 54. Volvox globator — after Klein and Janet 55. Life-history of Monocystis — after Biitschli 56. Life-history of Gregarina — after Biitschli 57. End-to-end union of Gregarines — after Frenzel 58. Life-history of Coccidium — after Schaudinn 59. Diagram of Protomyxa aurantiaca — after Haeckel 60. Formation of shell in a simple Foraminifer — after Dreyer 61. A pelagic Foraminifer — Hastigerina {Globigerina) murrayi — after Brady ....... 62. The trumpet-shaped ciliated Infusorian, Stentor — after Stein LIST OF ILLUSTRATIONS XV FIG. 63. 64. 65. 66. 67. 68. 69. 70. 71- 72. 73- 74- 75. 76. 77- 78. 79- 80. 8i. 82. 83- 84. 85. 86. 87. 88. 89. 90. 91. 92. 93 Optical section of a Radiolarian (Actinomma) — after Haeckel A Monad Infusorian — after Saville Kent Diagram of the structure of Noctiluca . Spore -formation in Noctiluca — after Roule Glossina palpalis, tsetse fly . . . Life-cycle of Plasmodium vivax . Trypanosoma gambiense .... Colonial Infusorian — Ophrydium sessile — after Saville Kent A colonial flagellate Infusorian — Proterosponia haeckelii — after Saville Kent .... Simple sponge (Ascetta primordialis) — after Haeckel A sponge colony ..... Sponge spicules ..... Section of a sponge — after F. E. Schulze Diagram showing types of canal system — after Korschelt and Heider. The flagellate regions are dark throughout, the mesogloea is dotted, the arrows show the direction of the currents. All the figures represent cross-sections through the wall ...... Diagram of sponge structure .... Development of Sycandra raphanus — after F. E. Schulze Diagrammatic representation of development of Oscarella lobularis — after Heider Diagram of early fixed stage of sponge . A, Young Dicyema — after Whitman. B, Female Orthonectid {Rhopalura giardii) — after Julin Salinella — after Frenzel ..... Diagram of Coelenterate structure, endoderm darker through out ...... Colony of Hydradinia on back of a Buccinum shell tenanted by a hermit-crab ..... Diagram of a typical Hydrozoon polyp — after Allman Diagram of stinging-cells or cnidoblasts, the one to the right undischarged ...... Hydra hanging from water-weed — after Greene Minute structure of Hydra — after T. J . Parkier and Jickeli Development of Hydra — after Brauer . Bougainvillea — after Allman .... Structure of a Swimming-bell or Medusoid, Obelia geniculata budded off from a Companularian Hydroid Structure of a Medusoid — after Allman , Surface view of A urelia — from Romanes h PAGE 135 138 141 143 145 146 148 154 154 156 157 161 162 163 165 165 169 170 171 173 174 177 179 181 182 183 185 XVI LIST OF ILLUSTRATIONS FIG. FAG 94. Vertical section of i4wr^/ia — after Claus . . . j86 95. Diagram of life -history of ^wr^/io — after Haeckel . . 188 96. Lucernaria — after Korotneff . . . . .189 97. Diagram of Lucernaria — after Allman . . .189 98. External appearance of Tealia crassicornis . . . 191 99. Vertical section of a sea-anemone — after Andres . .192 100. Section through sea-anemone (across arrow in Figure 99) — after Andres. ...... 193 loi. Z, Diagrammatic section of Zoantharian ; ^, of Alcyonarian — after Chun . . . . . . 194 102. The formation of a coral shell (Astroides) — after Pfurtscheller 196 103. Structure of Antipatharians ..... 197 104. Diagrams of Types of Alcyonaria — after Hickson . .198 105. Corallium rubrum, a corner of a colony — after Lacaze- Duthiers . . . . . • • 199 106. Alcyonarian spicules ...... 200 107. Diagram of a gymnoblastic Hydroid — after Allman . . 202 108. Graptolites ....... 202 109. Hydroids — after Hincks ..... 203 no. Campanularian Hydroid — after Allman . . . 204 111. Diagram of a Ctenophore — after Chun . . . 206 112. Hydroctena. Amedusoid with hints of Ctenophore structure 207 113. Commensalism of sea-anemones and hermit-crab . . 208 114. Portions of excretory system of flat-worms . . . 211 115. Diagram of Turbellarian — after Lang . . . .213 116. Structure of liver fluke — after Sommer. From ventral sur- face. The branched gut (g.) and the lateral nerve {l.n.) are shown to the left, the branches of the excretory vessel (e.v.) to the right ..... 117. Reproductive organs of liver fluke — after Sommer . 118. Life-history of liver fluke — after Thomas 119. Diagram of life-cycle of liver fluke .... 120. Male and female Bilharzia — Schistosomum hcsmatobium — after Looss ....... 121. Front end of the head of TcBnia solium .... 122. Diagram of reproductive organs in Cestode joint — con- structed from Leuckart ..... 123. Life-history of T^nm so/mw — after Leuckart 124. Diagram of life-history of Tania solium 125. Diagrams of bladder-worms ..... 126. Diagrammatic longitudinal section of a Nemertine [Amphi- porus lactifloreus), dorsal view — after M'Intosh. . 231 216 217 219 221 222 225 226 227 228 230 LIST OF ILLUSTRATIONS XVll riG. PAGE 127. Transverse section of the Nemertine Drepanophorus latus — after Biirger . . . • • .232 128. Transverse section of a simple Nemertine {Carinella) — after Biirger ....... 233 129. Cross-section through Ascaris ..... 237 130. Diagram of the structure of a male Nematode . . . 238 131. Trichinae in muscle, about to be encapsuled — after Leuckart 241 132. Trichinae in muscle, encapsuled. There may be 12,000 in a gramme of pig's muscle. After Leuckart . . 241 133. Earthworms ....... 246 134. Anterior region of earthworm — after Hering . . . 247 135. Transverse section of earthworm .... 251 136. Reproductive organs of earthworm — after Hering . . 253 137. Stages in the development of earthworm — after Wilson . 255 138. Arenicola marina . . . . . • '2 57 139. Anterior part of nervous system in Arenicola — after Vogt and Yung ....... 259 140. Dissection of lob-worm from dorsal surface . . . 260 141. Cross-section of Arenicola — after Cosmovici . . . 261 142. Diagram showing structure of a Trochosphere of Eupomatus — after Shearer ...... 263 143. Development of Polygordius — after Fraipont . . • 265 144. Parapodium of " Heteronereis " of Nereis pelagica — after Ehlers ....... 267 145. Free-living Polychaete {Nereis cuUrifera) . . . 268 146. Sex dimorphism in Bonellia viridis . . . • .269 147. Transverse section of leech — after Bourne . . • 274 148. Alimentary system of leech — after Moquin-Tandon . . 275 149. Dissection of leech — after Bourne .... 276 150. A nephridium of leech — after Boiume .... 277 151. Development of Sagitta — after O. Hertwig. Illustrating formation of a body cavity by pockets from the archen- teron ; also the early separation of reproductive cells . 281 152. Diagram of a Rotifer {Hydatina) — after Plate . . 283 153. Actinotrocha or larva of Phoronis — after Masterman . 285 154. Phoronis . . . . .... . . 285 155. Diagram of an Ectoproctous Polyzoon {Plumatella) . . 286 156. Interior of Brachiopod shell, showing calcareous support for the " arms " — after Davidson .... 287 157. Pluteus larva of Ophiuroid, with rudiment of adult — after Johannes Miiller ...... 290 158. Starfish ........ 293 XVlll LIST OF ILLUSTRATIONS FIG , PAGE 159. Dissection of A sterias rubens. From a specimen . . 294 160. Diagrammatic cross-section of starfish arm — after Ludwig . 296 161. Regeneration of a starfish from a separated arm — so-called " comet form "....,. 297 162. Ventral surface of disc of an Ophiuroid {Ophiothrix fragilis) — after Gegenbaur . . . . . .299 163. Apical disc of sea-urchin . . . . . 301 164. Dissection of sea-urchin ...... 302 165. Spicules of Holothurians — after Semon . . . 306 166. A small sea-cucumber ...... 307 167. Dissection of Holothurian {Holothuria tuhulosa) from the ventral surface ...... 309 168. Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea — after Milnes Marshall . 311 169. General appearance of a stalked Crinoid (P^n/acnmfs) . 313 170. Stages in development of Echinoderms — after Selenka . 316 171. Appendages of Norway lobster ..... 324 172. Central nervous system of the crayfish . . . 325 173. Section of compound eye of Mysis vulgaris — after Grenacher 326 174. An ommatidium of a compound eye . . . .327 175. Longitudinal section of lobster, showing some of the organs . 329 176. Male reproductive organs of crayfish — after Huxley . 332 177. Female reproductive organs of crayfish — after Suckow . 333 178. Section through the egg of Astacus after the completion of segmentation — after Reichenbach . . -334 179. Longitudinal section of later embryo of Astacus — after Reichenbach ...... 335 180. Section through cephalothorax of a crab — after Pearson . 336 181. Dorsal aspect of swimming crab (Portunus) . . . 337 182. Dorsal aspect of shore crab (Carcinus). . . . 337 183. Ventral aspect of female shore crab .... 338 184. Dorsal surface of Apus cancriformis — from Bronn's " Thierreich "...... 339 185. Daphnia . . . . . . . .341 186. Cypris ........ 342 187. Cyan's, side view, after removal of one valve — after Zenker . 342 188. Cyclops type ....... 343 189. Two barnacles hanging from a ship / . . . 344 190. Acorn-shell [Balanus tintinnabulum) — after Darwin . . 346 191. Development of Sacculina — after Delage. (Not drawn to scale) ....... 347 192. Sacculina as a parasite on a female crab — after Delage . 348 LIST OF ILLUSTRATIONS XIX FIG. 193. Nehalia — after Sars ...... 194. Anaspides — after Caiman ..... 195. An Amphipod [Caprella linearis) .... 196. Dorsal view of a wood-louse {Oniscus) — after Webb and Sillem ....... 197. Hermit-crab withdrawn from its shell. The anterior ap- pendages are broken off . 198. Mysis flexuosa, from side ..... 199. Nervous system of shore-crab {Carcinus mcenas) — after Bethe 200. Partial peripheral segmentation of the ovum — characteristic of Arthropods ...... 201. Zoaea of common shore-crab {Carcinus mcenas) — after Faxon 202. External form of Peripatus — after Balfour 203. Dissection of Peripatus — after Balfour 204. Embryos of Peripatus capensis, showing closure of blasto pore and curvature of embryo — after Korschelt and Heider 205. A millipede 206. A centipede 207. Mouth-parts and poison claws of a centipede 208. Mouth-parts of a millipede 209. Female cockroach (P. orientalis) 209A. Male cockroach (P. orientalis) 210. Mouth-parts of cockroach 211. Ventral aspect of male cockroach with the wings extended An imaginary median line has been inserted 212. Leg of cockroach ..... 213. Transverse section of insect — after Packard . 214. Head and mouth-parts of bee — after Cheshire 215. Nervous system of bee — after Cheshire 216. Food canal of bee — in part after Cheshire 217. Hive-bees and the cells in which they develop 218. Mouth-parts of mosquito — after Nuttall and Shipley 219. Young may- fly or ephemerid — after Eaton 220. Life-histories of insects . 221. Life-history of the silk-moth {Bombyx fnori) 222. A typical caterpillar 223. Development of blow-fly {Calliphora erythrocephala) — after Thompson Lowne .... 224. Fly about to emerge from pupa-case — after Hayek 225. Anufida maritima (after Imms), one of the primitive wing less Collembola ...•., PAGE 349 349 350 351 352 353 355 357 358 362 363 364 366 366 367 368 370 370 371 373 374 375 378 380 381 383 386 387 398 399 400 401 402 404 XX LIST OF ILLUSTRATIONS FIG. PAGK 226. Silver fish {Lepisma saccharina), one of the Thysanura . 405 227. Acerentomon, a very primitive insect .... 408 228. Mosquito — after Nuttall and Shipley . . . .411 229. Scorpion, ventral surface . . . . .413 230. Scorpion, dorsal surface . . . • • -415 231. Sex dimorphism in a spider (iVg/?/ii7a mgm) — after Vinson . 418 232. Dissection of My gale from the ventral surface — after Cuvier . 419 233. Section of lung-book — after Macleod .... 420 234. Follicle-mite (greatly enlarged) .... 423 235. Itch-mite {Sarcoptes scahiei) (greatly enlarged) . . 423 236. Tick [Ixodes riduvius, female), dorsal surface (after Wheler), showing the oval shield ..... 424 237. Tick {Ixodes riduvius, female), ventral surface — after Wheler ....... 424 238. Mouth-parts of a tick ...... 425 239. Limulus or King-crab, ventral aspect .... 427 240. Limulus or King-crab, dorsal aspect .... 428 241. Yowng Limulus — after Walcott. .... 429 242. Trilobite (Conocephalites) — after Barrande . . . 430 243. Vertical cross-section of a Trilobite (Calymene) — after Walcott . . . . • • -430 244. Under-surface of a Trilobite — after Beecher . . -431 245. Sea-spider {Pycnogonum littorale), from the dorsal surface . 432 246. Male of Nymphon — after Sars ..... 432 247. Ideal mollusc — after Ray Lankester .... 434 248. Stages in molluscan development .... 435 249. Roman snail (Helix pomatia) ..... 436 250. Vertical section of the shell of a species of Helix . . 437 251. Dissection of snail ...... 442 252. Reproductive organs of Wg/j;t /jowa^za — after Meisenheimer . 443 253. Snail (//e/jA;^owai!Ja) laying its eggs — after Meisenheimer . 444 254. Diagram of larva of Paludina — after Erlanger . . 445 255. The fresh-water mussel (C7mo) ..... 447 256. Structure of Anodonta — after Rankin .... 45i 257. Glochidium larva of the fresh-water mussel . . . 454 258. Side view of Se^m — after Jatta .... 456 259. External appearance of a squid {Loligo) . . . 458 260. Diagram of the structure of Sepia — mainly after Pelseneer . 459 261. Diagram of circulatory and excretory systems in a Decapod like Sepia — after Pelseneer .... 463 262. Male of Argonauta (after Jatta), showing " hectocotylus " arm ; compare Fig. 9, showing the female . . 464 LIST OF ILLUSTRATIONS XXI FIG. 263. Bunch of Sepia eggs attached to plant — after Jatta 264. Common buckie {Buccinum undatum) . 265. Bivalve {Panopcea norvegica), showing siphons 266. Nudibranch {Dendronotus arbor escens), showing dorsal out growths forming adaptive gills ... 267. Ventral surface of Patella vulgata — after Forbes and Hanley 268. Chiton — after Pretre ..... 269. Dorsal view of nervous system of Acanthochiton — after Pelseneer ....•• 270. Anatomy of Chiton ..... 271. A Pteropod [Cymhulia peronii), showing the wing-like ex pansions (pteropodial lobes) of the mid-foot 272. Stages in moUuscan development 273. Proneomenia. Nervous system — from Hubrecht 274. Section of shell of Nautilus — after Lendenfeld 275. The Pearly Nautilus {Nautilus pompilius) — after Owen 276. Section of the shell of Spirula .... 277. Spirula, a small Decapod cuttlefish 278. Male of Balanoglossus {Dolichoglossus) kowalevskii — after Bateson ...... 279. Dissection of Balanoglossus .... 280. Transverse section through gill-slit region of Ptychodera minuta — after Spengel .... 281. Direct development of Dolichoglossus — after Bateson 282. Tomaria larva, from the side — after Spengel . 283. Piece of a colony of Cephalo discus, showing the tubes in habited by the animals — after Ridewood 284. An individual Cephalodiscus — after Ridewood 285. Dissection of Ascidian — after Herdman 286. Diagram of Ascidian — after Herdman 287. Young embryo of Ascidian (Clavelina) — after Van Beneden and Julin ...... 288. Embryo of Clavelina — modified after SeeUger 289. Part of a colony of Botryllus, showing two individuals em bedded in a gelatinous matrix and with a common exhalant aperture . . .^ . 290. Asexual reproduction in Salpa .... 291. " Nurse " of Doliolum mUlleri — after Uljanin. 292. Sexual individual of Doliolum millleri — after Uljanin 293. Diagram of Salpa africana .... 294. Structure of Appendicular ia — after Herdman 295. Lateral view of Amphioxus — after Ray Lankester . PAGE 465 466 467 468 469 471 471 472 475 476 478 483 484 486 487 491 492 493 494 496 497 497 5or 504 506 507 508 509 510 510 511 512 516 XXll LIST OF ILLUSTRATIONS FIG . PAGE 296. Transverse section through pharyngeal region of Amphioxus — after Ray Lankester . . . . .518 297. Development of atrial chamber in Amphioxus — after Lankester and Willey . . . . .519 298. Sections through embryos of Amphioxus, to illustrate de- velopment of body cavity — after Hatschek . . 522 299. The nephvidia. of Amphioxus — after Boveri . . . 524 300. Small portions of excretory organs of Amphioxus (A) and the Polychaete Phyllodoce (B) — after Goodrich . '525 301. Early stages in the development of Amphioxus — after Hatschek . . . . . . .526 302. Larval Amphioxus, from the right side — after Willey . 527 303. Diagrammatic section through a Vertebrate animal . -536 304. Ideal fore- and hind-limb — after Gegenbaur . . . 538 305. Partial section of a Vertebrate brain (diagrammatic) . . 540 306. Vertical section of the pineal eye in an embryo of Spheno- don — after Dendy . . . . . -541 307. Diagram illustrating the development of the nervous system in an Elasmobranch fish ..... 544 308. Diagrammatic section of spinal cord .... 546 309. Diagram of spinal cord of man, thoracic region — after Johnston ....... 546 310. Diagram showing development of ear in a Vertebrate (cartilaginous fish) ...... 548 311. Diagram showing the ear and related parts in a young cat . 549 312. Diagram of the eye ...... 550 313. Development of the eye — after Balfour and Hertwig . 551 314. Diagram of embryonic pharynx in a Mammal — after Goette . 555 315. Origin of lungs, liver, and pancreas in the chick — after Goette ....... 556 316. Section through a young newt ..... 557 317. Section through Elasmobranch embryo — after Ziegler . 559 318. Blood corpuscles . . . . . . .560 319. Diagram showing the valves {VA.) common in veins . .561 320. Diagram of circulation — after Leunis .... 563 321. Development of excretory system of Vertebrate — in part after Boveri ...... 566 322. Urinogenital system of Chordata Vertebrata . . . 567 323. Mammalian ovum — after Hertwig .... 572 324. Median longitudinal section of anterior region of Myxine — after Retzius and Parker ..... 576 325. Respiratory system of hag, from ventral surface . . 577 LIST OF ILLUSTRATIONS XXlll FIG. PAGE 326. Bdellostoma stouti (Californian hag), enveloped in sheath of mucus — after Bashford Dean .... 579 327. The lamprey {Petromyzon marinus) .... 580 328. Longitudinal vertical section of anterior end of larval lamprey ....... 583 329. Restored skeleton of Palceospondylus gunni — after Traquair 585 330. Pterichthys milleri. Lateral view — restored by Traquair . 586 331. Diagram of the placoid scale of an Elasmobranch fish . 590 332. Diagram of the " soft " scales of a Bony Fish . . -591 333. Under surface of skull and arches of skate — after W. K. Parker ....... 592 334. Side view of skate's skull — after W. K. Parker . . 593 333. Skeleton of skate — from a preparation . . -594 336. Dissection of nerves of skate ..... 596 337. Side view of chief cranial nerves of Elasmobranchs — slightly modified from Cossar Ewart . . . 600 338. Spiral valve of skate — after T. J. Parker . . . 601 339. Heart and adjacent vessels of skate — in part after Monro . 602 340. Upper part of the dorsal aorta in the skate — after Monro . 603 341. Urinogenital organs of male skate .... 604 342. Urinogenital organs of female skate — in part after Monro . 605 343. Elasmobranch development — after Balfour . . . 606 344. Embryo dogfish in egg-case ("mermaid's purse") which has been cut open to show contents . . . 608 345. External features of Acanthias vulgaris . . . 610 346. Acanthias vulgaris. Longitudinal section showing organs . 611 347. Side view of brain of Acanthias vulgaris — after Purser . 612 348. Dorsal view of brain ol Acanthias vulgaris — after Purser . 613 349. Arterial system of Acanthias — after O'Donoghue . . 614 350. The haddock ....... 616 351. External characters of a Teleostean — a carp {Cyprinus carpio) — after Leunis . . . . .617 352. Lines of growth on salmon's scale — from J. Arthur Hutton . 6x8 353. Diagram of the zigzag myotomes in a fish . . . 6x8 354. Caudal vertebra of haddock ..... 6x9 355. Disarticulated skull of cod ..... 620 356. Pectoral girdle and fin of cod . . . . .621 357. Section of a Teleostean gill ..... 622 358. Diagram of Teleostean circulation — after Nuhn . . 624 359. The early development of the salmon . . . 625 360. Development of eel — after Schmidt .... 629 361. Young skate — from Beard ..... 633 Xxiv LIST OF ILLUSTRATIONS PAGE FIG. 362. Lateral view of dogfish {Scyllium catulus) . . . 634 363. Polypterus bichir . ,....• 637 364. Larva of Polypterus (after Budgett), li inch in length . 637 365. Sturgeon {A cipenser sturio) ..... 638 366. Bony pike {Lepidosteus osseus) ..... 639 367. The Queensland lung-fish {Ceratodus forsteri) . . . 642 368. Protopterus, the African mud-fish .... 643 369. Larva of Protopterus — after Budgett .... 643 370. Lepidosiren (after Grahana Kerr), showing (Pc.F.) pectoral fin and the tufted pelvic fin {Pv.F.) of the mature male . . • • • • 371. 'Laxvdi oi Lepidosiren — after Graham Kerr 372. Skeleton of Ceratodus fin — from Gegenbaur . 373. The edible frog (Rana esculenta) 374. Vertebral column and pelvic gurdle of bull-frog 375. Skull of frog — upper and lower surface — after W. K. Parker 376. Pectoral girdle of Rana esculenta — after Ecker 377. Skeleton of frog. The half of the pectoral girdle, and fore and hind-limb of the right side, are not shown 378. Side view of frog's pelvis — after Ecker 379. Brain of frog — after Wiedersheim 380. Nervous system of frog — after Ecker . 381. Arterial system of frog . 382. Venous system of frog . 383. Urinogenital system of male edible frog — after Ecker . 384. Urinogenital system of female frog— after Ecker 385. Division of frog's ovum — after Ecker . 386. Section of frog embryo— after Ziegler's model and Marshall 387. Dissection of tadpole— after Milnes Marshall and Bles 388. Life-history of a frog— after Brehm . 389. Male and female of the Crested Newt {Triton or Molge cr (status) . . . • • 390. The Axolotl {Amhly stoma tigrinum) 391. Proteus anguineus .... 392. Limbless subterranean Amphibian {Ichthyophis) 393. Caecilian {Ichthyophis) with eggs— after Sarasin 394. Foetal membranes in Amniota — after Roule . 395. Sphenodon punctatus. The New Zealand "Lizard" 396. Lateral view of brain of Sphenodon punctatus — after Osawa 397. External appearance of tortoise 398. Skull of turtle ...••• 399. Carapace of tortoise ..... 645 645 647 651 653 654 655 656 657 658 659 663 664 668 668 671 672 674 676 678 678 679 679 681 684 685 685 687 687 688 LIST OF ILLUSTRATIONS XXV FIG. PAGE 400. Pectoral girdle of a Chelonian ..... 689 401. Internal view of the plastron of the Greek tortoise . . 690 402. Scales on ventral surface of plastron of Greek tortoise . 690 403. Internal view of skeleton of tortoise .... 691 404. Dissection of Chelonian heart — after Huxley . . .691 405. Heart and associated vessels of tortoise — after Nuhn . 692 406. Hyoid apparatus of a Chelonian .... 692 407. Roof of the skull of a lizard (Varanid) . . . 695 408. Side view of skull of Lacerta — after W. K. Parker . . 696 409. Pectoral girdle of a lizard ..... 698 410. Heart and associated vessels of a lizard — after Nuhn . . 699 411. Lung of ChamcsleG vulgaris, showing air-sacs — after Wieders- heim ........ 700 .}i2. Slow-woxia {Angiiis fragilis), alimhlesslizard . . . 702 413. Anterior view of Python's vertebra .... 704 414. Posterior view of Python's vertebra .... 704 415. Snake's head — after Nuhn ..... 705 416. Side view of skull of non-poisonous Pythonid snake . . 706 417. Side view of skull of a poisonous snake .... 707 418. Skull of grass-snake — from W. K. Parker . . . 708 419. Lower siurface of skull of a young crocodile . . . 710 420. First vertebra of crocodile . . . . .711 421. First five cervical vertebrae of crocodile . . .712 422. Cervical vertebra of crocodile . . . . .713 423. Crocodile's skull : dorsal surface .... 714 424. Pectoral girdle of crocodile . . . . .715 425. Half of the pelvic girdle of a young crocodile . . 715 426. Origin of amnion and allantois — after Balfour . .718 427. Vertical section through backbone and ribs of a Chelonian and a Mammal — in part after Jaekel . . .721 428. Position of organs in a bird — after Selenka . . . 727 429. Anterior view of a dorsal vertebra of an ostrich . . 728 430. Fore-limb and hind-limb compared . . . 729 431. D iagrammatic section of young bird — after Gadow . . 730 432. A falcon ........ 731 433. Young heaxded grifhn (Gypaetus barbattis) — after Nitzsch . 732 434 . Young feather and filoplume — after Nitzsch . . . 733 435. Diagram showing a stage in the development of a feather . 733 436. Types of feathers ...... 735 437. Parts of a feather — after Nitzsch .... 736 438. Entire skeleton of condor, showing the relative positions of the chief bones . . . . . . 738 XXVI LIST OF ILLUSTRATIONS FIG. 439. Disarticulation of bird's skull — after Gadow. Membrane bones shaded 440. Vertebral or upper part of the rib of a bird 441. Under surface of gull's skull 442. Pectoral girdle and breastbone of an eagle 443. Wing of dove .... 444. Side view of pelvis of cassowary 445. Bones of hind-limb of eagle 446. Pelvic girdle and hind-limb of a fowl . 447. Brain of pigeon .... 448. Diagrammatic section of cloaca of male bird — after Gadow 449. Heart and arterial system of pigeon 450. Heart and venous system of pigeon 451. Diagram of air-sacs of a pigeon. 452. Female urinogenital organs of pigeon 453. Male urinogenital organs of pigeon 454. Pectoral girdle and sternum of swan 455. Position of wings in pigeon at maximum elevation — from Marey ..... 456. Wings coming down — from Marey 457. Wings completely depressed — from Marey 458. Stages in development of chick — after Marshall 459. Diagrammatic section of egg — after Allen Thomson 460. Diagrammatic section of embryo — after Kennel 461. Restoration of Archaeopteryx — by permission of Mr. W. P Pycraft ..... 462. Flightless Apteryx or Kiwi of New Zealand . 463. Pectoral girdle of an ostrich 464. Hesperornis — after Marsh 465. Pectoral girdle and adjacent parts in a gibbon 466. Diagram of skull bones (partly after Flower and Weber) the membrane bones shaded 467. Occipital region of cat's skull . 468. Fore-limb and shoulder-girdle and hind-limb of rabbit 469. Atlas vertebra of a Mammal, side view 470. Front view of Mammalian atlas 471. Side view of the axis of a Mammal 472. Side view of rabbit's skull 473. Upper surface of rabbit's skull . 474. Under surface of rabbit's skull . 475. Skull of capybara 476. Scapula of a rabbit PAGE 739 740 741 742 743 744 744 745 746 748 750 751 753 755 755 757 758 759 759 766 767 768 769 771 772 773 779 781 782 785 786 787 788 789 790 791 792 794 LIST OF ILLUSTRATIONS XXVU FIG. 477- 478. 479- 480. 481. 482. 483- 484. 485. 486. 487. 488. 489. 490. 491. 492. 493- 494. 495- 496. 497- 498. 499. 500. 501. 502. 503. 504 505 506 507 508 509 510 511 512 after Claude a section, with Pelvic girdle and terminal vertebrae of a gibbon Dorsal view of rabbit's brain ..... Under surface of rabbit's brain — after Krause Median vertical section through Mammal's brain — after Edinger .... Diagram of the alimentary tract of a rabbit Diagram of caecum in rabbit Blood corpuscles . Duodenum of rabbit — from Krause, in part Bernard Circulatory system of the rabbit Structure of mammalian heart . Vertical section through rabbit's head — from help from Parker's Zootomy and Krause Urinogenital organs of male rabbit Urinogenital organs of female rabbit . Dentition of a dog Segmentation of rabbit's ovum — after Van Beneden . Development of hedgehog. Three early stages — after Hubrecht ....... Embryo of Perameles with its foetal membranes — after Hill . Two stages in segmented ovum of hedgehog — after Hubrecht Development of foetal membranes — after Hertwig Diagram of foetal membranes — after Turner . View of embryo, with its foetal membranes — after Kennel . Diagram of foetal membranes in a rabbit — in part after Bonnet Pectoral girdle of Echidna Pelvis of Echidna. Lower jaw of kangaroo . , Foot of young kangaroo . Side view of sheep's skull , Stomach of sheep — from Leunis . Side view of lower part of pony's fore-leg . Side view of ankle and foot of horse . Side view of horse's skull . Feet of horse and its predecessors — from Neumayr . Vertical almost median section through elephant's skull . Left fore -limb of Balcenoptera ..... . Fore-limb of whale {Megaptera longimana) — after Struthers . . Pelvis and hind-limb of Greenland whale (Balcena) — after Struthers .....•• PAGE 796 798 798 799 801 802 803 803 804 805 808 810 810 817 818 819 820 821 822 823 826 828 834 836 838 842 849 850 853 853 855 855 858 861 861 863 XXVlll LIST OF ILLUSTRATIONS FIG. 513. Vertebra, rib, and sternum of Balcenoptera — from specimen in Anatomical Museum, Edinburgh 514. Lower jaw of a rodent .... 515. Skull of tiger, lateral view 516. Lower surface of dog's skull 517. Side view of the brain of a dog . 518. Fore -arm and hand of the mole 519. Outline of a bat's wing .... 520. Skeleton of one of the large bats (Megachiroptera) 521. Skull of orang-utan .... 522. Skull of gorilla ..... 523. Fore-limb of a small monkey . 524. Skeleton of male gorilla .... 525. Hind-limb of a gibbon .... 526. Restoration of head of Pithecanthropus erectus — after MacGregor ..... 527. Restoration of head of Homo neanderthalensis — after MacGregor ..... 528. Zoo-geographical regions of the earth's surface PAGE 865 866 868 869 870 875 877 878 886 887 888 889 891 892 893 906 C/5 < Pi pq W > C/3 Pi w > ^2; O BIRDS. Flying Birds. Running Birds. Placentals. MAMMALS. Marsupials. Monotremes. Snakes. Lizards. REPTILES. Crocodiles. Tortoises. FISHES. Dipnoi. Bony Fishes. "Ganoids." Elasmobranchs. AMPHIBIANS. Newt. Frog. CYCLOSTOxMES. Lamprey. Hag-fish. LANCELETS. TUNICATES. Insects. Arachnids. Myriopods. Peripatus. ARTHROPODS. Crustaceans. BALANOGLOSSUS. ANNELIDS. "WORMS." UNSEGMENTED WORMS. Cuttle-fishes. Gasteropods. MOLLUSCS. Bivalves. Feather-stars. Brittle-stars. Star- fish. ECHINODERMS. Sea-urchins. Sea-cucumbers. Ctenophores. Jelly-fish. Sea-anemones. CCELENTERA. Medusoids and Hydroids. Corals. SPONGES. Infusorians. Rhizopods. Sporozoa. SIMPLEST ANIMALS. OUTLINES OF ZOOLOGY CHAPTER I GENERAL SURVEY OF THE ANIMAL KINGDOM In beginning the study of Zoology, it is natural and useful to try to get a bird's-eye view of the " Animal Kingdom." Without this, one is apt to miss the plan in studying the details. But the survey can be of little service unless the student has the actual animals in his mind's eye. Vertebrates, or Backboned Animals Mammals. — We begin our survey with the animals which are anatomically most like man — the monkeys. But neither we nor the monkeys are separated by any structural gulf from the other four-limbed, hair-bearing animals, to which Lamarck gave the name of Mammals. For although there are many different types of Mammals — such as monkeys and men ; horses, cattle, and other hoofed quad- rupeds ; cats, dogs, and bears ; rats, mice, and other rodents ; hedgehogs, shrews ^ and moles, and so on — the common possession of certain characters unites them all in one class, readily distinguishable from^Birds and Reptiles. These distinctive characters include the milk-giving of the mother mammals, the growth of hair on the skin, the general presence of convolutions on the front part of the brain, the occurrence of a muscular partition or diaphragm between the chest and the abdomen, and so on, as we shall 2 GENERAL SURVEY OF THE ANIMAL KINGDOM afterwards notice in detail. Most mammals are suited for life on land, but diverse types, such as seals, whales, and sea-cows, have taken to the water. In another direction the bats are markedly adapted for aerial life. Among the mammalian characteristics of great import- ance are those which relate to the bearing of young, and even a brief consideration of these shows that some mammals are distinguished from others by differences deeper than those which separate whales from carnivores, or rodents from bats. These deep differences may be stated briefly as follows : — {a) Before birth most young mammals are very closely united (by a complex structure Fig. I. — Duckmole (Ornithorhynchus). called the placenta) to the mothers who bear them, {b) But this close connection between mother and unborn young is of rare occurrence, or only hinted at, in the pouched animals or Marsupials, which bring forth their young in a peculiarly helpless condition, as it were prematurely, and in most cases place them in an external pouch, within which they are sheltered and nourished, (c) In the Australian duckmole and its two relatives, the placental connection is quite absent, for these animals lay eggs as birds and most reptiles do. These differences and others relating to structure warrant the division of Mammals into three sub- classes : — (a) Eutheria, Monodelphia, or Placentals — those in which there is a close (placental) union between the unborn embryo and its mother, e.g. Ungulates, Carnivores, Monkeys. BIRDS (6) Metatheria, Didelphia, or Marsupials — the prematurely bearing, usually pouch-possessing kangaroos, opossums, etc. (c) Prototheria, Ornithodelphia, or Monotremes— the egg-laying duckmole (Ornithorhynchus), Echidna, and Proechidna. Fig. 2. — Phenacodus, a primitive extinct Mammal. — After Cope. Birds. — There can be no hesitation as to the class which ranks next to Mammals. For Birds are in most respects as highly developed as Mammals, though in a different direc- tion. They are character- ised by their feathers and wings, and many other adaptations for flight, by their high temperature, by the frequent spongi- ness and hollowness of their bones, by the tend- ency to fusion in many parts of the skeleton, by the absence of teeth in modern forms, by the fixedness of the lungs and their association with numerous air sacs, and so on. Fig. 3. — Extinct moa and modern kiwi. — After Carus Sterne. But here again different grades must be distinguished — (i) There is the vast majority — the flying birds, with a breast -bone keel or carina, to 4 GENERAL SURVEY OF THE ANIMAL KINGDOM which the muscles used in flight are in part attached (Carinatae) ; (2) there is the small minority of running birds (ostriches, emu, cassowary, kiwi, and extinct moa), with wings incapable of flight, and with no keel (Ratita?) ; and (3) there is an extinct type, Archcpopteryx, with markedly reptihan affinities. Reptiles. — There are no close relationships between Birds and Mammals, but the old-fashioned Monotremes have some markedly reptilian features, and so have some aberrant living birds, such as the Hoatzin and the Tinamou. Moreover, when we consider the extinct Mammals and Birds, we perceive other resemblances linking the two highest classes to the Reptiles. Fig. 4. — Crocodiles. Reptiles do not form a compact class, but rather an assemblage of classes. In other words, the types of Reptile differ much more widely from one another than do the types of Bird or Mammal. Nowadays there are five dis- tinct types :— the crocodilians, the unique New Zealand " lizard " (Sphenodon), the lizards proper, the snakes, and the tortoises. But the number of types is greatly increased when we take account of the entirely extinct saurians, who had their golden age in the inconceivably distant past. The Reptiles which we know nowadays are scaly-skinned animals ; they resemble Birds and Mammals in having during embryonic life two important " foetal membranes " (the amnion and the allantois), and in never having gills ; they differ from them in being " cold-blooded," and in many other ways. AMPHIBIANS Amphibians. — The Amphibians, such as frogs and newts, were once regarded — e.g. by Cuvier — as naked Reptiles, but a more accurate classification has linked them rather to the Fishes. Thus Huxley grouped Birds and Reptiles together as Sauropsida ; Amphibians and Fishes together as Ichthyopsida — for reasons which will be after- wards stated. Amphibians mark the transition from Fig. 5. — Salamander, an Amphibian. aquatic life, habitual among Fishes, to terrestrial life, habitual among Reptiles ; for while almost all Amphibians have gills — in their youth at least — all the adults have lungs, and some retain the gills as well. In having limbs which are fingered and toed, and thus very different from fins, they resemble Reptiles. But the two fcetal membranes characteristic of the embryonic life of higher Vertebrates are not present in Amphibian embryos, and the general absence of an exoskeleton in modern forms is noteworthy. Fishes. — The members of this class are as markedly adapted to life in the water as birds to life in the air. The very muscular posterior region of the body usually forms Fig. 6. — Queensland Dipnoan {Ceratodus). the locomotor organ, and we say that a fish swims by bending and straightening its tail. The limbs have the form of paired fins — that is, they are limbs without digits. There are also unpaired median fins supported by fin rays. All have permanent gills borne by bony or gristly arches. 6 GENERAL SURVEY OF THE ANIMAL KINGDOM There is an exoskeleton of scales, and the skin also bears numerous glandular cells and sensory structures. In many ways Fishes are allied to Amphibians, especially if we include among Fishes three peculiar forms, known as Dipnoi, which show the beginning of a three-chambered heart, and have a lung as well as gills. Ordinary Fishes have a two-chambered heart, containing only impure blood, which is driven to the gills, whence, purified, it passes directly to the body. Apart from the divergent Dipnoi, there are two great orders of Fishes — the cartilaginous Elasmobranchs, such as shark and skate ; and the Teleosteans or bony fishes, such as cod, herring, salmon, eel, and sole. There are several smaller orders of great importance, some of which, e.g. the sturgeons, are often called " Ganoids." Primitive Vertebrates. — Under this title we include — (i) the Roundmouths or Cyclostomata ; (2) the lancelets or Cephalochorda ; (3) the Tunicates, some of which are Fig. 7. — Lancelet, Amphioxus. — After Haeckel. called sea-squirts ; and (4), with much hesitation, several strange forms, especially Balanoglossus, which exhibit structures suggestive of affinity with Vertebrates. The Cyclostomata, represented by the lamprey (Petro- myzon) and the hag {Myxine), and some other forms, probably including an interesting fossil known as Palceo- spondylus, are sometimes ranked with fishes under the title Marsipobranchii. But they have no definitely developed jaws, no paired fins, no scales, and are in other ways more primitive. The lancelets or Cephalochorda are even simpler in their general structure (see Fig. 7). Thus there is an absence of Umbs, skull, jaws, well-defined brain, heart, and some other structures. The vertebral column is represented by an unsegmented (or unvertebrated) rod, called the noto- chord, which in higher animals (except Cyclostomes and some fishes) is a transitory embryonic organ afterwards replaced by the backbone. CHARACTERISTICS OF VERTEBRATES The Tunicata or Urochorda are remarkable forms, the majority of which degenerate after larval life (Fig. 8). In the larvae of all, and in a few adults which are neither peculiarly specialised nor degenerate, we recognise some of the fundamental characters of Vertebrates. Thus there is a dorsal supporting axis (or notochord) in the tail region, a dorsal nervous system, gill - clefts opening from the pharynx to the exterior, a simple ventral heart, and so on. Of Balanoglossus and its allies (Hemichorda or Enteropneusta) it is still difficult to speak with confidence. The possession of gill - clefts, the dorsal position of an important part of the nervous system, the occurrence of a short supporting structure on the anterior dorsal surface of the pharynx, and other features, have led many to place them at the base of the Vertebrate series. Fig. 8. — Ascidian or sea - squirt. — After Haeckel. Characteristics of Vertebrata. — At this stage, having reached the base of the Vertebrate series, we may seek to define a Vertebrate animal, and to contrast it with Invertebrate forms. The distinction is a very old one, for even Aristotle distinguished mammals, birds, reptiles, amphibians, and fishes as " blood-holding," from cuttle-fish, shell-bearing animals, crustaceans, insects, etc., which he regarded as " bloodless." He was, indeed, mistaken about the bloodlessness, but the distinctiveness of the higher animals first mentioned has been recognised by all subsequent naturalists, though it was first precisely expressed in 1797 by Lamarck. Yet it is no longer possible to draw a boundary line between Verte- brates and Invertebrates with that firmness of hand which characterised the early or, indeed, the pre-Darwinian classifications. We now know — (i) that Fishes and Cyclostomata do not form the base of the Vertebrate series, for the lancelets and the Tunicates must also be in- cluded in the Vertebrate alliance ; (2) that Balanoglossus, Cephalodiscus, and some other forms, have several Vertebrate-like characteristics ; (3) that some of the Invertebrates, especially the ChaBtopod worms, show some hints of affinities with Vertebrates. The limits of the Vertebrate alliance have been widened, and though the recognition of their characteristics has become more definite, not less so, the apartness of the sub-kingdom has disappeared- 8 GENERAL SURVEY OF THE ANIMAL KINGDOM It does not matter much whether we retain the familiar title Verte- brata, or adopt that of Chordata, provided that we recognise — (i) that it is among Fishes first that separate vertebral bodies appear in the supporting dorsal axis of the body ; (2) that, as a characteristic, the backbone is less important than the notochord, which precedes it in the history alike of the race and of the individual. Nor need we object to the popular title backboned, if we recognise that the adjective " bony " is first applicable among Fishes, and not to all of these. The essential characters of Vertebrates may be summed up in the following table, where they are contrasted, somewhat negatively, with what is true of Invertebrates : — " Backboneless," Invertebrate OR Non-Chordate. " Backboned," Vertebrate OR Chordate. If there is a nerve-cord, it is ventral. No internal dorsal axis. No gill-slits. The eye is usually derived directly from the skin. The heart, if present, is dorsal. The central nervous system — brain and spinal cord — is dorsal and iubttlar. There is a dorsal supporting axis or notochord, which is in most cases replaced by a backbone. Gill-slits or visceral clefts open from the sides of the pharynx to the exterior. In fishes, and at least young amphi- bians, they are associated with gills, and are useful in respiration ; in higher forms they are transitory and functionless, except when modified into other structures. The essential parts of the c^-e are formed by an outgrowth from the brain. The heart is ventral. Invertebrates, or Backboneless Animals Molluscs. — If we take the concentration of the nervous system as a useful criterion, the highest backboneless animals are the Molluscs. This series of forms includes Bivalves, such as cockle and mussel, oyster and clam ; Gasteropods, such as snail and slug, periwinkle and whelk ; Cephalopods, such as octopus and pearly nautilus. Unlike Vertebrates, and such Invertebrates as Insects and Crustaceans, Molluscs are without segments and without appendages. A muscular protrusion of the ventral surface, known as the " foot," serves in the majority as an organ of locomotion. In most cases a single or double fold of skin, called the " mantle," makes a protective shell. The nervous system has three chief pairs of nerve centres or ganglia. In many cases there are very characteristic free-swimming larval stages. ARTHROPODS Fig. 9. — Cephalopod (paper nautilus, female). Arthropods. — This large series includes Crustaceans, Myriopods, Insects, Spiders, and other forms, which have segmented bilaterally symmetrical bodies and jointed Fir,. 10. — Fresh- water crayfish {Astacus), a Crustacean. — After Huxlev. Fig. II. — a, Caterpillar ; b, pupa ; c, butterfly. 10 GENERAL SURVEY OF THE ANIMAL KINGDOM Fig. 12. — Spider. appendages. The skin produces an external, not-living cuticle, the organic part of which is a substance called chitin, associated in Crustaceans with carbonate of lime. The nervous system con- sists of a dorsal brain, connected, by a nerve - ring around the gullet, with a ventral chain of ganglia. Echinoderms. — This is a well- defined series, including star-fishes, brittle-stars, sea - urchins, sea- cucumbers, and feather-stars. The symmetry of the adult is usually radial, though that of the larva is bilateral. A peculiar system, known as the water-vascular system, is characteristic, and is turned to various uses, as in locomotion and respiration. There is a marked tend- ency to deposition of lime in the tissues. The de velopment is strangely circuitous or " indirect." Segmented "worms." — It is hopeless at present to arrange with any definiteness those heterogeneous forms to which the title " worm " is given. For this title is little more than a name for a shape ^ assumed by animals of varied nature who began to move head foremost and to acquire sides. There is no class of " worms," but an as- semblage — a mob — not yet reduced to order. It seems useful, however, to separate those which are ringed or segmented from those which are unsegmented. The former are often called Annelids, and include twQ chief classes ; — ^ Fig. 13. — Crinoid or feather-star. WORMS I I (i) Chaetopoda or Bristle-footed worms, e.g. earthworm and lob- worm ; and (2) Hirudinea or Leeches. Fig. 14. — Earthworm. Unsegmented * ' worms. ' ' — These differ from the higher " worms " in the absence of true segments and appendages, and resemble them in their bilateral symmetry. There is a motley lot : — the free-living Turbellarians or Planarians ; the parasitic Trematodes or Flukes ; the parasitic Cestodes or Tape-worms ; the Nemer- teans or Ribbon-worms ; the frequently parasitic Nematodes or Thread-worms ; and several smaller classes. As to some other groups, such as the sea-mats (Polyzoa or Bryozoa), the lamp-shells (Brachiopoda), the worm-like Sipunculids, and the wheel- animalcules or Rotifers, we must confess that they are still incertce sedis. But the general fact is not without interest, that in the midst of the well-defined classes of Invertebrates there lies, as it were, a pool from which many streams of life have flowed ; for among the heterogeneous " worms " we may find in diverse types affinities with Arthropods, Molluscs, Echinoderms, and even Vertebrates. Contrast of Ccelomate and Coelenterate. — At this stage we may notice that in all the above forms the typical symmetry is bilateral (in Echinoderms, the superficial radial symmetry belongs only to the adults) ; that in most types a body cavity or ccelom is developed ; that the embryo consists of three germinal layers (external Fig. 15. — Bladderworm stage of a Cestode. — After Leuckart. a, Early stage with head inverted. b, Later stage with head everted. 12 GENERAL SURVEY OF THE ANIMAL KINGDOM ectoderm or epiblast, internal endoderni or hypoblast lining the gut, and a median mesoderm or mesoblast lining the body cavity). In the next two classes (Ccelentera and Sponges) the conditions are different, as may be expressed in the following table : — Sponges and Ccelentera. Higher Animals (Ccelomata). There is no body cavity. There is but one cavity, that of the food canal. Except in ctenophores, there is no definite middle layer of cells (meso- derm), but rather a middle jelly (mesoglcea), and the embryo is diploblastic. The radial symmetry of the gastrula embryo is usually retained in the adult, and the longitudinal (oral- aboral) axis of the adult corresponds to the long axis of the gastrula. There is a body cavity or ccelom be- tween the food canal and the body- wall. But this is often incipient, or degenerate. There is a distinct middle layer of cells (mesoderm) between the external ectoderm and the internal endo- derni. The embryo is triploblastic. The adults are usually bilateral, in some cases asymmetrical, in echinoderms superficially radial. Ccelentera. — This series includes jelly-fishes, sea- anemones, corals, zoophytes, and the like, most of which are Fig. 1 6. — Sea-anemones on back of hermit-crab. — After Andres. equipped with stinging cells, by means of which they paralyse their prey. All but a few are marine. The body may be a tubular polyp, or a more or less bell-like " medu- PROTOZOA 13 soid," and in some cases the two forms are included in one life cycle. Budding is very common, and many of the sedentary forms — " corals " — have shells of lime. Porifera. — Sponges, or Porifera, are the simplest many- celled animals. In the simplest forms, the body is a tubular, two-layered sac, with numerous inhalant pores by which water passes in, with a central cavity lined by cells bearing lashes or flagella, and with an exhalant aperture. But budding, folding, and other complications arise, and there is almost always a skeleton, calcareous, siliceous, or " horny." Apart from one family (Spongillidae), all sponges are marine. Contrast of Metazoa and Protozoa.— All the animals hitherto mentioned have bodies built up of many cells,i but there are other animals, each of which consists of a single cell. These simplest animals are called Protozoa. Every animal hitherto mentioned, from mammal or bird to sponge, develops, when reproduction takes its usual course, from a fertilised egg-cell. This egg-cell or ovum divides and redivides, and the daughter cells cohere and are differentiated to form a " body." But the Protozoa form no "body"; they remain (with few exceptions) single cells, and when they divide, the daughter cells almost invariably go apart as independent organisms. Here, then, is the greatest gulf which we have hitherto noticed — that between multicellular animals (Metazoa) and unicellular animals (Protozoa). But the gulf was bridged, and traces of the bridge remain. For — (a) there are a few Protozoa which form loose colonies of cells, and (6) there are a few multicellular animals of great simplicity. Protozoa. — The Pro- tozoa remain single cells, with few exceptions. Thus they form no " body " ; and necessarily, therefore, they have no organs in the ordinary sense. They illustrate the beginnings of sexual reproduction, and they are not subject to natural death in the same degree as Metazoa are. The series includes — 1 A cell may be defined as a unit corpuscle or unit area of living matter, typically controlled by a single nucleus. Fig. 17. — Fossil Foraminifera (Nummulites) in limestone. — After Zittel. 14 GENERAL SURVEY OF THE ANIMAL KINGDOM (a) Rhizopods, with outflowing threads or processes of hving matter, e.g. the chalk-forming Foraminifera (Fig. 17). (b) Infusorians, with actively moving lashes of living matter. (c) Sporozoa, parasitic forms, usually without either lashes or out- flowing processes. Note on Classification We always group together in our mind those impressions which are hke one another. In this hes the beginning of all classification, whether that of the child, the savage, or the zoologist. For there are many possible classifications, varying according to their purpose, according to the points of similarity which have been selected as important. Thus we may classify animals according to their habitats or their diet, without taking any thought of their structure. But a strictly zoological classification is one which seeks to show the blood-relationships of animals, to group together those whose affinities are shown by their being like one another in architecture or structure. It must, therefore, be based on the results of comparative anatomy — technically speaking, on " homologies," i.e. resemblances in funda- mental structure and in mode of development. Whales must not be ranked with fishes, nor bats with birds. To a classification based on structural resemblances, two corrobora- tions are of value, from embryology and from paleontology. On the one hand, the development of the forms in question must be studied : thus no one dreamed that a Tunicate was a Vertebrate until its life- history was worked out. On the other hand, the past history must be inquired into : thus the affinity between Birds and Reptiles is confirmed by a knowledge of the extinct forms. In classification it is convenient to recognise certain grades or degrees of resemblance, which are spoken of as species, genera, famihes, orders, classes, and so on. To give an illustration, all the tigers are said to form the species Felis tigris, of the genus Felis, in the family FeHdae, in the order Carnivora, within the class Mammalia. The resemblances of all tigers are exceedingly close ; well marked, but not so close, are the resera blances between tigers, lions, jaguars, pumas, cats, etc., which form the genus Felis ; broader still are the resemblances between all members of the cat family Felidae ; still wider those between cats, dogs, bears, and seals, which form the order Carnivora ; and lastly, there are the general resemblances of structure which bind Mammals together in contrast to Birds or Reptiles, though all are included in the series or phylum Vertebrata. It must be understood that the real things are the individual animals, and that a species includes all those individuals who resemble one another so closely that we feel we need a specific name applicable to them all. And as resemblances which seem important to one naturalist may seem trivial to others, there are often wide differences of opinion as to the number of species which a genus contains. But while no rigid definition can be given of a species, certain common-sense considerations should be borne in mind : — GENEALOGICAL TREE 15 I. No naturalist now believes, as Linnaeus did, in the fixity of species ; we believe, on the contrary, that one form has given rise to another. At the same time, the common characteristics on the strength of which we deem it warrantable to give a name to a group of individuals, must Gregoj-ine^ Fig. 18. — Diagrammatic expression of classification in a genealogical tree. B indicates possible position of Balano- glossus, D of Dipnoi, S of Sphenodpn or Hatteria, P of Peripatus. not be markedly fluctuating. The specific characters should exhibit a certain degree of constancy from one generation to another. 2. Sometimes a minute character, such as the shape of a tooth or the marking of a scale, is so constantly characteristic of a group of indi- viduals that it may be safely used as the index of more important i6 GENERAL SURVEY OF THE ANIMAL KINGDOM characters. On the other hand, the distinction between one species and another should always be greater than any difference between the members of a family (using the word family here to mean the progeny of a pair). For no one would divide mankind into species according to the colour of eyes or hair, as this might lead to the absurd conclusion that two brothers belonged to different species. Thus it is often doubly unsatisfactory when a species is established on the strength of a single specimen — (a) because the constancy of the specific character is unde- termined ; {b) because the variations within the limits of the family have not been observed. Indeed, it has happened that one species has been made out of a male, and another out of its mate. 3. Although cases are known where members of different species have paired and brought forth fertile hybrids, this is not usual. The members of a species are fertile inter se, but not usually with members of other species. In fact, the distinctness of species has largely depended on a restriction of the range of fertility. Tabular Survey. — {For Future Reference) METAZOA CHORDATA Mammalia. AVES. Reptilia. Amphibia. Pisces. Cyclostomata. I'Eutheria. -| Metatheria. Marsupials. \ Prototheria. Monotremes. Oviparous. {Carinatae. Keeled flying birds. Odontolcae. Extinct toothed birds. Ratitae. Keel-less running birds. Extinct reptile-like birds. I'Crocodilia. Crocodiles and alligators. Ophidia. Snakes. Lacertilia. Lizards. "j Rhynchocephalia. Sphenodon. I Chelonia. Tortoises and turtles. ^Extinct Classes. Anura. Tail-less frogs and toads. Urodela. Tailed newts. Gymnophiona, e.g. CcBcilia. I Labyrinthodonts and other extinct \ Amphibians. /"Dipnoi. Mud-fishes. -[ Teleostomi. Bony fishes, etc. V. Elasniobranchii. Cartilaginous fishes., / Hag-fish [Myxine), and Lamprey \ {Petromyzon). \ Hi Cephalochorda. Amphioxus. Urochorda. Tunicates. Hemichorda. Balanoglossus, Cephalodiscus. 'J) o C/2 Oh c ni ■*-» Ti O o 2'^ ) .2 « r- O-r Invertebrates ^1 METAZOA NON-CHORDATA MOLLUSCA. i Cephalopoda. Cuttle-fishes. Gasteropoda. Snails. Lamellibranchiata. Bivalves. Two smaller classes : — Scaphopoda and Solenogastres. Arthropod.'^. ECHINODERMA. /Arachnoidea. Spiders, scorpions, mites. I Insecta. I Myriopoda. Centipedes and millipedes. I- Prototracheata. Peripatus. Crustacea. Palaeostraca : — Trilobites, Eurypterids, and King-crabs. ,^Some smaller classes. ^Crinoidea. Feather-stars. (Cystoids and Blastoids, extinct. ) Ophiuroidea. Brittle-stars. -' Asteroidea. Star-fishes. I Echinoidea. Sea-urchins. V Holothuroidea. Sea-cucumbers. " Worms." Chaetopoda. Bristle worms. ^ . ,. i Discophora. Leeches. I Annelids or Some smaller classes. / ^nnuiata. I" Brachiopoda. Lamp-shells. I Polyzoa, e.g. Sea-mat (Flusira). \ Sipunculoidea, e.g. Siptinciilus. Nematoda. Thread-worms. Acanthocephala. Nemertea. Ribbon-worms. Rotifera. Wheel-animalcules. t Cestoda. Tape-worms. ^ I Trematoda. Flukes. '• > VTurbellaria. Planarians. J CcELENTERA. / Ctenophora, e.g. Beroe. Actinozoa or Anthozoa. Sea-anemones. Alcyonarians and re- lated corals. Scyphoniedusae or Acraspeda. Jelly-fishes. Hydrozoa. Zoophytes and medusoids. PORIFERA. Sponges. Calcareous and non-calcareous. PROTOZOA Infusoria. Rhizopoda. Sporozoa. Simplest forms of animal life. i8 GENERAL SURVEY OF THE ANIMAL KINGDOM INVERTEBRATE ANIMALS 19 • M o q^ O ^ — ^ a* ^ 03 OS l-l > oi u o 6 in ■a a, o <« ^ So "Si i . 6 "^ o • o 6 c 0! ™ « y CO « 2 • o [? ^ a m . • ^^ ^ . — .S rt "a; IH (b r; «C/3 q re S3 .t£o ■2 3 '.S y.„ J2'3 "y en tn " y ^2 3 They build up crude, chemically simple food material into living or complex substances ; they convert the kinetic energy of sunlight into the potential chemical energy of these com- plex substances ; they are characteris- tically reducers (of carbon dioxide), ex- pend comparatively little energy in motion or external work, are predomin- antly passive, and show in the vital changes associated with their living matter or protoplasm, a relative pre- ponderance of constructive, up-build- ing, or " anabolic " processes. a) < S Z < b tn H (/■■ 5 u H y <; ft! <: 3 U ."2 1 tn in _y Ih 2 6 -a y y M-i >. y They obtain the requisite carbon from starch, sugar, fat, etc., made by plants or by other animals. They obtain the requisite nitrogen from nitrogenous compounds, not sim- pler than proteins, made by other or- ganisms. Most of them are known to get rid of nitrogenous waste products. They have very rarely any chloro- phyll. The component cells often have no very definite cell walls, rarely have them of material demonstrably different from the cell substance, and almost never show any trace of cellulose. bo 3 6 re i-i 3 .0 re "^ .y M-i *■+-• .in *i-i .2 " in 2 '> re •^ _3 -O y ■73 •'- i-> "y re S y .3 They utilise food material already worked up by plants or by other animals ; they convert this potential energy into kinetic energy in locomo- tion and external work ; they are characteristically oxidisers, are pre- dominantly active, and show in the vital changes associated with their living matter or protoplasm, a relative preponderance of disruptive, down- breaking, or " katabolic " processes. t/i Z H Cu U y fc) S !/3 Some Protozoa and para- sites simply absorb. Some green Protozoa (etc. ?) seem to be able to utilise carbon dioxide as plants do {holopkytic). Again, some Protozoa are probably able to feed like plants. A few, e.g. some Protozoa, have chlorophyll. Others, e.g. the fresh-water sponge, Hydra viridis, and Convo- luta, have symbiotic .\lgae with chlorophyll. Cellulose seems to occur in some Infusorians, and forms most of the tunic or cuticle of the passive sea- squirts or ascidians. ■ 26 THE FUNCTIONS OF ANIMALS the aid of catalysts, which may be compared to lubricants. Thus pure hydrogen does not combine with oxygen except at very high temperatures, when it does so with explosive violence ; but the two gases can be got to com- bine at a low temperature by the introduction of finely- divided platinum as a catalyst. The essential features of a catalyst are that it is not used up in the course of the reaction which it promotes, that accordingly a small quantity of catalyst is sufficient for a great mass of reactant, and that only reactions in which energy is set free can be promoted — a catalyst cannot supply energy. Within living cells oxidations are aided by catalysts of different kinds. In the first place, the enormous surfaces of the microscopic and ultra-microscopic constituents of the protoplasm are of importance, since in their neigh- bourhood are local peculiarities of concentration and electrical forces. In the second place, there are certain fairly simple organic substances, such as glutathione, which are easily oxidised and reduced again, and in their transmutations assist in the oxidation of more resistant compounds. In the third place, there are catalysts of a special type, peculiar to living organisms, known as enzymes or ferments. The chemical nature of these enzymes is obscure ; they are very sensitive to changes in the medium in which they act (acidity, etc.), and are easily destroyed ; and they are markedly specific. Each enzyme — a large number being known — catalyses only one reaction or a small group of reactions. Their activity is not restricted to oxidations ; as we shall see, they play a most important part in the processes of digestion. Chief functions of the animal body. — There are two master activities in animals, those of muscular and of nervous structures ; the other vital processes, always ex- cepting growth and reproduction, are subservient to these. Let us now consider these master and subsidiary functions, as they occur in some higher organism, such as man. Nervous activities. — Life has been described as action and reaction between the organism and its environment, and it is evident that an animal must in some way become aware of surrounding influences. An external influence stimulates a sensory cell or its n nu Wr\-^ / HI su ^ h.c ,^" N.C SPG ii±f^v:^MI A ^\ i^F 21. — Diagrams of reflex actions. — Modified from Bayliss's Principles of General Physiology. I. In a sea-anemone a stimulus from a sensory nerve-cell or neurone (S.C.) at the surface (SU.) may pass directly bv a sensory fibre (S.F.) to a muscle (MU.). II. In some cases, e.g. in the earthworm, the stimulus passes into the ventral nerve-cord (N.C.) and directly affects a motor nerve-cell or neurone (M.C.). Thence an impulse travels by a motor fibre (M.F.) to the muscle (MU.). III. More usually in the earthworm and similar types there are associative neurones (A.) interpolated in the nerve-cord (N.C.) between the branches of the sensory nerve fibre (S.F.) and the dendrites of the motor neurones (M.C.). IV. In Vertebrates from a nerve-ending (N.E.) on the surface (SU.) the stimulus passes by a sensory fibre to the sensory neurones in the spinal ganglia (on the dorsal roots of the spinal nerves) and thence into the spinal cord (SP.C). Associative or internuncial neurones (A.) connect the branches of the sensory nerve-fibre with the den- drites of the motor neurone (M.C.). The impulse travels along the axis cylinder or motor fibre to the muscle (MU.). 28 THE FUNCTIONS OF ANIMALS ending, and a message travels by a sensory fibre to the nerve-cord. The inner end of the sensory fibre is con- nected with the branches or dendrites of an associative (communicating or internuncial) cell. Thence the message is passed, still within the cord, to the dendrites of a motor nerve-cell. Thence an efferent impulse travels down the axis-cylinder or motor nerve-fibre of the motor neurone to an ending on a muscle fibre, which is thus commanded to contract. The whole nervous system is essentially a con- nected series of such reflex-arcs, all intricately joined up with one another. The passage of an impulse along a nerve-fibre may be compared to the passage of a flame along a train of gun- powder. In each case the strength of the initial stimulus does not affect the strength of the transmitted stimulus — the " all-or-none law " is obeyed. In each case the passage of an impulse prevents a second impulse from following close behind it, although the " refractory period " of a nerve-fibre, during which recovery or repair takes place, is extremely short. Oxidation is involved in both cases, though the oxygen consumption of nerves is small. But whereas the train of gunpowder serves to transmit a difference of temperature from one place to another, the nerve-fibre transmits a difference of electric potential from point to point. There are two chief kinds of stimuli which are trans- mitted to the central nervous system — stimuli from without the body, which make the organism aware of changes in its environment ; and stimuli from within the body, which make it aware of the dispositions of its organs, e.g. the stimuli transmitted by the afferent nerves of the muscles, tendons, etc. The chief functions of the nervous system are, then, to make the animal aware of its environment and to co- ordinate and integrate all its bodily functions and activities. As we ascend in the scale, we find that in addition the brain possesses, to an increasing extent, the power of correlating present and past experiences and of originating or inhibiting action in accordance with this correlation. In whatever part there is activity, there is necessarily waste of complex substances and some degree of exhaustion ; and it is CONTRACTION OF MUSCLE Zg interesting to notice, as a triumph of histological technique, that Hodge, Gustav Mann, and others have succeeded in demonstrating in nerve cells the structural results (cellular collapse, etc.) of fatigue, and that in such diverse types as bee, frog, bird, and dog. Muscular activity. — The movements of a unicellular animal are due to the contractility of the living matter, or of special parts of the cell, such as lashes or cilia. In sponges specially contractile cells begin to appear ; in most higher animals such cells are aggregated to form the muscles. There are two distinct types of muscle, with some inter- mediate forms. The most highly developed is the cross- striated or skeletal muscle, which typically consists of numerous fine transparent tubes or fibres, each invested by a sheath or sarcolemma, while the whole muscle is surrounded by connective tissue. It usually runs from one part of the skeleton to another, and is fastened to the skeleton by tendons or sinews. It is controlled by motor nerves, which may bring about a sharp " twitch " con- traction, a powerful maintained contraction (tetanus), or a steady *' postural " state of tension (tonus). The fibres of smooth or involuntary muscle are of an attenuated spindle shape ; their contractions are more sluggish, and they frequently encircle hollow organs without being bound to the hard parts of the body. They are not under voluntary control, though they have their motor nerves and take part in reflexes ; but they have some inde- pendence of the nervous system, and can maintain a tonic tautness or even carry on a rhythmic series of contractions automatically. Muscle displays to some extent the phenomena of the all-or-none law and the refractory period. When a muscle contracts, usually under a stimulus propagated along a motor nerve, it and each of its fibres becomes shorter and broader. The contraction of the fibres is itself a physical rather than a chemical pheno- menon, like a change in the state of a spiral spring. In the actual contraction there is no using up of oxygen or output of CO2, but lactic acid is set free from glycogen within the muscle, and this acts by its acidity on the fibres, causing them to contract. Relaxation takes place when the lactic acid is neutralised, chiefly by bases set free from combina- 30 THE FUNCTIONS OF ANIMALS tion with the muscle proteins. When oxygen is available, about one-fifth of the neutraUsed lactic acid is burnt to CO2 and water, and so much energy is obtained by this reaction that the rest of the lactic acid can be resynthesised into glycogen. The process of contraction is therefore much more economical in the presence of oxygen, and fatigue is deferred. Besides the chemical change and the change of shape, there are also changes of " electric poten- tial " associated with each contraction. Beside muscular movement we must rank ciliary, amoeboid, and epithelial movement. Under the last heading are included active non- amceboid contractions and expansions of covering cells. Digestion: — The energy expended in work or in growth is balanced by the energy of the food-stuffs : — proteins, carbo- hydrates, fats, water, and salts, in varying proportions. In some of the lower animals, such as sponges, the food particles are engulfed by certain cells with which they come in contact, and digested within these cells {intracellular digestion). In most cases, however, the food is digested within the food canal, by ferments made by the secretory cells of the gut or of associated glands. The peculiarity of these ferments is that a small quantity can act upon a large mass of material without itself undergoing any apparent change. However digestion be effected, it means dissolving the food and making it diffusible. In a higher vertebrate there are many steps. {a) The first ferment to affect the food, masticated by the teeth and moistened by the saliva, is the p^alin of the salivary juice, which changes starch into sugar. The juice is formed or secreted by various salivary glands around the mouth. {b) The food is swallowed, and passes down the gullet to the stomach, where it is mixed with the gastric juice secreted by glands situated in the walls. These walls are also muscular, and their contractions churn the food and mix it with the juice. In the juice there is some free hydrochloric acid and a ferment called pepsin : these act together in turning proteins into peptones. The juice has also a slight solvent effect on fat, and the acid on the carbohydrates. (c) The semi -digested food, as it passes from the stomach into the small intestine, is called chyme, and on this other juices act. Of these the most important is the secretion of the pancreas, which contains various ferments, e.g. trypsin, and affects all the different kinds of organic food. It continues the work of the stomach, changing proteins into peptones and peptones into much simpler compounds such as amino-acids ; it continues the work of the salivary juice, changing DIGESTION 31 starch into sugar ; it also emulsifies the fat, dividing the globules into extremely small drops, which it tends to saponify or spUt into fatty acids and gl3'cerine. {d) Into the beginning of the small intestine the bile from the liver also flows, but it is not of great digestive importance, being partly of the nature of a waste product. It has a very important action in lowering surface tension so that the fatty constituents of the chyme can form a finely divided emulsion, readily attacked by the digestive ferment from the pancreas, and it also aids in the absorption of the digested fat by the cells lining the intestine. In some animals it is said to have shght power of converting starch into sugar ; by its alkalinity it helps the action of the trypsin of the pancreas (which, unlike pepsin, acts in an alkaline fluid) ; and it is said to have various other qualities. (e) In addition to the liver and the pancreas, there are on the walls of the small intestine a great number of small glands, which secrete a juice which seconds the pancreatic juice ; this contains the ferment erepsin, which completes the sphtting of peptones into amino-acids, and ferments which spht the more complex sugars, such as cane sugar. The digested material is in part absorbed into the blood, and the mass of food, still being digested, is passed along the small intestine by means of the miiscular contraction of the walls known as peristaltic action. It reaches the large intestine, and its reaction is now distinctly acid by reason of the acid fermentation of the contents. The walls of the large intestine contain glands similar to those of the small intestine, and the digestive processes are completed, while absorption of water also goes on ; so that by the time the mass has reached the rectum, it is semi-sohd, and is known as faeces. These contain the indigestible and undigested remnants of the food, especially cellulose ; residues of the secretions of the digestive glands ; and enormous numbers of bacteria (mostly dead) from the large intestine, with the products of their activity. The digestive processes of Invertebrates are, in a general way, similar : for instance, an alternation of acid and alkaline reactions in digestion is common, though not so well marked as in Vertebrates. There is, however, much adaptation to the diet, both in the structure of the alimentary canal and in the ferments secreted. Un- necessary ferments are dispensed with ; for example, many carnivorous insects have lost the power to digest starch. New ferments are evolved -to digest substances of which other groups can make no use ; thus the clothes- moth larva can digest the very resistant keratin of hair. Cellulose and some other insoluble carbohydrates, which constitute the hard parts of plants, can be digested by some protozoa, and by snails and perhaps a few other families ; but in many groups, for instance in some wood-boring 3^ THE FUNCTIONS OF ANIMALS insects, these substances are split up not by secreted ferments but by the action of symbiotic microbes within the food-canal, or in close relation to it. Absorption. — But the food must not only be rendered soluble and diffusible, it must be carried to the different parts of the body, and there incorporated into the hungry cells. It is carried by the blood stream, and in part also by what are called lymph vessels, which contain a clear fluid resembling blood minus red blood corpuscles. Absorption begins in the stomach by direct osmosis into the capillaries or fine branches of blood vessels in its walls, and a similar absorption, especially of water, takes place along the whole of the digestive tract. But lining the intestine there are delicate projections called villi ; they contain capillaries belonging to the portal system (blood vessels going to the liver), and small vessels known as lacteals connected with lymph spaces in the wall of the intestine. The lacteals lead into a longitudinal lymph vessel or thoracic duct, which opens into the junction of the left jugular and left subclavian veins at the root of the neck. The contents of the duct in a fasting animal are clear ; after a meal they become milky ; the change is due to the matters discharged into it by the lacteals. It is probable that nearly all the fat of a meal is absorbed from the intestines by the lacteals, but it is not certain in what measure, if at all, this is true of the other dissolved food-stuffs; the greater part certainly passes into the capillaries of the portal system, which are contained in the villi. The digested protein, chiefly in the form of amino-acids, passes into the blood of the portal vein, either directly or through the intermediary of leucocytes, which flock to the intestine when protein food is being digested. Functions of the liver. — The absorbed products of the digestion of proteins and carbohydrates are carried from the intestine to the liver by the portal vein, which splits up into fine channels (sinusoids) in close connection with the liver cells. In digestion the more complex carbo- hydrates, such as starch, glycogen, and cane sugar, are split into molecules of simple sugars such as glucose. A large part of the glucose absorbed after a meal is stored in the liver in the form of glycogen ; the muscles of the body also contain glycogen, from which lactic acid is pro- duced, and as their glycogen stores are depleted they draw upon the glucose of the blood. One of the functions of the liver is to maintain the concentration of glucose in the blood at a constant level, by mobilising its glycogen stores as required. These equilibria are partly controlled by a substance (insulin) formed in the pancreas. DIGESTION 33 The end-products of the digestion of proteins are the amino-acids, which contain nitrogen in the form of amino or NHo groups. In the Hver these are spHt off to form ammonia, which combines with carbon dioxide to form ammonium carbonate. By removal of water, probably in the liver cells, this compound is converted into urea, which is excreted from the body by the kidneys. The nitrogen-free residues of the amino-acids have either a carbohydrate or a fatty character, and yield energy to the body by being oxidised. Although the fats absorbed from the intestine do not pass through the portal system directly, the liver plays a part in their metabolism. Some of the links of the long carbon-atom chains of the fatty acids are weakened, so that their subsequent oxidation is facilitated ; and fatty acids are also combined with phosphoric acid to form lecithin, which is more easily transported about the body than the true fats. There is no special organ for the regulation of the amount of fat ; the drops pass through the walls of the capillaries and are stored in connective tissue cells. The liver has many other functions, for example, in the preparation of the bile, which contains both valuable adjuvants to digestion and useless waste products ; it is, in short, the most important chemical clearing-house of the body. Many Invertebrates, such as Molluscs and Crustacea, possess a large digestive gland called the " liver " or hepatopancreas. It combines the function of the verte- brate pancreas, in secreting digestive enzymes, with that of the liver in storing absorbed food and probably in carrying out various chemical reactions and setting free waste products ; and very often there takes place intracellular digestion and assimilation of particles of food brought up from the alimentary canal proper — a function unknown in Vertebrates. The relative importance of these activities varies from group to group. Respiration. — There is another most important material to be noticed, namely, the oxygen which is absorbed from the air by the lungs. We may picture a lung as an elastic sponge-work of air chambers, with innumerable blood 3 34 THE FUNCTIONS OF ANIMALS capillaries in the walls, enclosed in an air-tight box, the chest, the size of which constantly and rhythmically varies. When we take in a breath, the size of the chest is increased, the air pressure within is lowered, and the air from without rushes down the windpipe until the pressure is equalised. The oxygen of this air combines with the coloured iron- containing protein called haemoglobin, contained in the red corpuscles of the blood, and is thus carried to all parts of the body. From the blood it passes to the tissues usually through the medium of the lymph. It is used in the tissues for oxidation. The carbon dioxide formed as a waste product is temporarily combined with bases set free from the blood proteins, especially haemoglobin, and so in time reaches the lungs. But as the partial pressure of the carbonic acid in the air is lower than it is in the serum, the gas escapes from the latter into the air chambers of the lungs. When the size of the chest is decreased, the pressure is increased, and the gas escapes by the mouth or nose until the pressure is equaUsed. Many very different types of respiratory organ are met with in the animal kingdom. Fundamentally, a respiratory organ is a region of the body-surface, usually either an in-folding or an out-folding, at which the external air or water is brought into close relation with either the body-fluids or with the cells themselves, so that diffusion of gases takes place readily. But different animals have solved the problem in different ways, and within each group many adaptations to the conditions of various environments may be demonstrated. There may be deep differences in the physiology of respiration between different phyla. In birds and mammals, which maintain a constant body temperature, there is no direct relation between the external temperature and the amount of oxygen consumed ; but in most cold- blooded animals metabolism increases with rising tempera- ture until the heat becomes harmful. It is usually found that the consumption of oxygen by the body is, within wide limits, independent of the concentration of oxygen in the external medium ; but in some Invertebrates, in which increase of size has not been balanced by the de- velopment of efficient respiratory and circulatory systems, EXCRETION 35 there is a regular relation between metabolism and the partial pressure of oxygen. It has been explained that muscle cells derive energy from the splitting of glycogen, without oxidation, though they effect an economy by using oxygen when they can. Many bacteria derive all their energy from such reactions ; and it has been suggested that some Invertebrates, such as intestinal worms, which normally live in a medium extremely poor in oxygen, do the same. But the most recent researches (of Slater and others) make it unlikely that any of the Metazoa have so fundamentally adapted their physiology that they can thrive all their lives without oxidations ; though undoubtedly many Invertebrates have an amazing power of surviving the absence of oxygen for long periods. Excretion. — We have seen that the blood carries the digested food to the various parts of the body, and that it is also the carrier of oxygen and of the waste carbon dioxide. But there is much waste resulting from tissue changes, which is not gaseous. It is cast into the blood stream by the tissues, and has to be got rid of in some way. This is effected by the kidneys, which are really filters introduced into the blood stream. But they are the most marvellous filters imaginable, and give us a good example of the intricacy of life processes. For the kidneys not only take out of the blood all the waste products that result from the metabolism of proteins and contain nitrogen, they also maintain the composition of the blood at its normal, rejecting any stuffs that vary from that normal, either qualitatively or quantitatively, doing this work according to laws quite different from the simple laws of diffusion or solubility : thus sugar and urea are about equally soluble, and yet the sugar is kept in the body, while the urea is cast out. Even substances as insoluble as resins are removed from the blood by the living cells of the kidneys. A considerable quantity of water, plus traces of salts, fats, etc., leaves the body by the skin, but its chief use is to protect, and to regulate the temperature by variations in the size of its blood vessels. Some special substances are 36 THE FUNCTIONS OF ANIMALS excreted into the alimentary canal in the bile or by the cells lining the large intestine. This completes our sketch — (a) of the process by which the food becomes available for the organism as fuel for the maintenance of its life energies, and (b) of the removal of the waste products which are formed as the ashes of life. Some organs have not been mentioned, such as the spleen, an accessory reservoir for blood, also an area for the multiplication of red blood corpuscles (fishes, newts, embryo-mammals) or for the destruction of worn- out corpuscles (mammals) ; and the various ** endocrine " glands, which make and pour into the blood specific substances called hormones, whose function it is to regulate the activity of cells in other parts of the body. Thus the thyroid glands form thyroxin, a general stimulant of metabolism. But what we have said is perhaps enough to convey a general idea of the processes of life in a higher animal. In conclusion, it is perhaps useful to remark that when in the course of further studies the student meets with organs which are called by the same name as those found in man or in Mammals, as, for example, the " liver " (^f the Molluscs, he must be careful not to suppose that the function of such a " liver " is the same as in Mammals, for comparatively little investigation into the physiology of the lower types of animal hfe has as yet been made. At the same time, he must clearly recognise that the great internal activities are in a general way the same in all animals ; thus respiration, whether accomplished by skin, or gills, or air-tubes, or lungs, by help of the red pigment (haemo- globin) of the blood, or of some pigment which is not red, or occurring without the presence of any blood at all, always means that oxygen is absorbed almost like a kind of food by the tissues, and that the carbon dioxide which results from the oxidation of part of the material of the tissues is removed. Modern Conception of Protoplasm The activities of animals are ultimately due to physical and chemical changes associated with the living matter or protoplasm. This is a mere truism. We do not know the nature of this living matter ; perhaps our most certain knowledge of it is, that in our brains its activity is associated with consciousness. PROTOPLASM 37 When more is known in regard to the chemistry and physics of Uving matter, it may be possible to bring vital phenomena more into line with the changes which are observed in inorganic things. At present, however, it is idle to deny that vital phenomena are things apart. Not even the simplest of them can be explained in terms of chemistry and physics. Even the passage of digested food from the gut to the blood vessels is more than ordinary physical osmosis ; it is modified by the fact that the cells are living. There are some processes going on in the body of which a complete account can be given in chemical and physical terms, but we cannot, at present at least, give in chemical and physical terms an adequate account of any distinctively vital action, nor of growth and development, nor of behaviour. But though we cannot analyse living matter, nor thoroughly explain the changes by which the material of the body breaks down or is built up, we can trace, by chemical analysis, how food passes through various trans- formations till it becomes a usable part of the living body, and we can also catch some of the waste products formed when muscles or other parts are active. Living protoplasm is in a colloidal state, i.e. ultra- microscopic solid particles and immiscible droplets are in suspension and free movement in a fluid. There is a complex mixture of proteins, carbohydrates, fats, and some inorganic constituents, and 70-90 per cent, of water. In this mixture there is a complex play of forces, such as those of surface-tension and electrical charge, and a great variety of chemical changes, summed up in the term " metabolism." Diflterent kinds of chemical changes go on in close prox- imity to one another, yet with some degree of separateness, like eddies in a stream. Perhaps the localisation of par- ticular processes within the cell depends on the deposition of more stable, less labile constituents, forming a sort of framework — the furniture of the living laboratory. When the substance of a cell is fixed and stained, it often shows an intricate reticular, fibrillar, or alveolar structure, but this seems to be mainly a post-mortem effect. However this may be, the cytoplasm of living cells is 38 THE FUNCTIONS OF ANIMALS certainly far from homogeneous. The complex colloidal system is delicately poised between two states, the one truly fluid, though viscous, the other truly solid, though gelatinous. The fluctuations between these ' gel ' and ' sol ' states are largely governed by local variations in concentra- tion of the electrically charged ions of acids, bases, and inorganic salts. Within the cell certain areas, the nucleus, for example, seem to be enclosed by invisibly thin semi- solid membranes, and a similar membrane, more easily studied, forms the external boundary of the cell. This " plasma membrane " exhibits the phenomenon of semi-permeability — that is to say, it permits the passage of some substances, but retains others. Molecules of water and of gases pass through readily, and so also do many organic substances of the class of the " fat-solvents," such as alcohol, ether, chloroform, benzene. All these soak into the substance of the plasma membrane and have a narcotic action, depressing the activity of the cell ; for, as Lillie has said, the plasma membrane is more than a mere partition : it is a sensitive intermediary between the cell and the external world. On the other hand, water- soluble organic substances, such as sugars and amino- acids and urea, and all inorganic salts, are held back by the healthy plasma membrane. But the plasma membrane is a most delicate structure, whose semi-permeable pro- perties are easily impaired ; for example, a pure solution of sodium chloride is harmful to most cells, but the addi- tion of a little calcium chloride '' balances " the solution and renders it harmless ; sea water is a perfectly balanced solution. The question of the permeability of the plasma membrane enters into almost every physiological problem, and will be referred to again. Generalising from his studies on colour sensation, Professor Hering was led to regard all life as an alterna- tion of two kinds of activity, both induced by stimulus, the one tending to storage, construction, assimilation of material, the other tending to explosion, disruption, dis- assimilation. Generalising from his studies on nervous activities, Professor Gaskell was led to regard all life as an alterna- tion of two processes, one of them a running down or dis- ANABOLISM AND KATABOLISM 39 ruption (katabolism), the other a winding up or construc- tion (anaboUsm). All physiologists are agreed that in life there is a twofold process of waste and repair, of discharge and restitution, of activity and recuperative rest. But there is no certainty as to the precise nature of this twofold process. CHAPTER III THE ELEMENTS OF STRUCTURE (Morphology) Animals may be studied alive or dead, in regard to their activities or in regard to their parts. We may ask how they Hve, or what they are made of ; we may investigate their functions or their structure. The study of hfe, activity, function, is physiology ; the study of parts, architecture, structure, is morphology. The first task of the morphologist is to describe structure (descriptive anatomy) ; the second is to compare the parts of one animal with those of another (comparative anatomy) ; the third is to try to state the " principles of morphology," or the laws of vital architecture. But just as the physiologist investigates life or activity at different levels, passing from his study, of the animal as a unity with certain habits, to consider it as an engine of organs, a web of tissues, a city of cells, and a whirlpool of living matter ; so the morphologist has to investigate the form of the whole animal, then in succession its organs, their component tissues, their component cells, and finally, the structure of protoplasm itself. The tasks of morphology and of physiology are parallel. Morphology thus includes not only the description of ex- ternal form, not only the anatomy of organs, but also that minute anatomy of tissues and cells and protoplasm which we call histology. Moreover, there is no real diff"erence between studying fossil animals which died and were buried countless years ago, and dissecting a modern frog. The anatomical palaeontologist is also a student of morphology. Finally, as the greater part of embryology consists in study- 40 FORM AND SYMMETRY 4I ing the anatomy and histology of an organism at various stages of its development, the work of the embryologist is also in the main morphological, though he has also to inform us, if he can, about the physiology of development. Morphology has been defined by Geddes as " the study of all the statical aspects of organisms," in contrast to physiology, which is concerned with their vital dynamics. In this chapter we shall follow the historical development of morphology, and work from the outside inwards. I. Form and symmetry. — The form of an animal is due to the interaction of two variables — the protoplasmic material which composes the organism, and the environ- ment which plays upon it. In some measure, an animal takes definite form as a mineral does : in both the shape is determined by the nature of the stuff and by the surround- ing influences. But the form of an animal is also aflPected by function, i.e. by action and reaction between the organism and its surroundings. As regards symmetry, animals may be distinguished as — {a) radially symmetrical ; [b) bilaterally symmetrical ; (c) asymmetrical. In a radially symmetrical animal, such as a jelly-fish, the body can be halved by a number of vertical planes — it is symmetrical around a median vertical axis. That is, it is the same all round, and has no right or left side. In a bilaterally symmetrical body, such as a worm's, there is but one plane through which the body can be halved. In an asymmetrical animal, such as a snail, accurate halving is im- possible. Radial symmetry is illustrated by simple Sponges, most Coelentera, and b)' many adult Echinoderms. As it is the rule in the two lowest classes of Metazoa, and as it is characteristic of the very common embryonic stage known as the gastrula (an oval or thimble -shaped sac consisting of two layers of cells), it is probably more primitive than the bilateral symmetry characteristic of most animals above Coelentera. Radial symmetry seems best suited for sedentary life, or for aimless floating and drifting. Bilateral symm^etry probably arose as it became advantageous for animals to move energetically^ and in definite direc- tions, to pursue their prey, avoid their enemies, and seek their mates. The formation of a " brain " is correlated with the habit of moving head foremost. Among many-celled animals, some worm type prob- ably deserves the credit of beginning the profitable habit of moving head foremost. Had some one not taken this step, we should never have known our right hand from our left. Axial gradient. — A physiological analogue to symmetry is to be found in the " axial gradients " studied by Child 42 THE ELEMENTS OF STRUCTURE and others. In a bilaterally symmetrical animal, such as a flat-worm, the head is the region of greatest physiological activity, e.g. most intense metabolism and greatest sus- ceptibility to external influences. Behind the head the activity decreases towards the relatively inert middle part of the body ; the tail is a secondary centre of activity, less intense than the head. It is possible to demonstrate these gradients, and the physiological dominance of one region over another, in many ways and in all types of animals, as well as in organs and sometimes in single cells. These studies provide a new point of view for the better under- standing of many evolutionary processes, especially for the nervous system, and of many problems of behaviour, regeneration, etc. 11. Organs. — We give this name to any well-defined part of an animal, such as heart or brain. The word sug- gests a piece of mechanism ; but the animal is more than a complex engine, and many organs have several different activities to which their visible structure gives little clue. Dijferentiation and integration of organs. — When we review the animal series, or study the development of an individual, we see that organs appear gradually. The gastrula cavity — the future stomach — is the first acquisition, though some would make out that it was primitively a brood-chamber. To begin with, it is a simple sac, but it soon becomes complicated by digestive and other out- growths. The progress of the individual, and of the race, is from apparent simplicity to obvious complexity. We also notice that before definite nervous organs appear there is diffuse irritability, before definite muscular organs appear there is diffuse contractility, and so on. In other words, functions come before organs. The attainment of organs implies specialisation of parts, or concentration of functions in particular areas of the body. If we contrast a frog with Hydra, one of the great facts in regard to the evolution of organs is illustrated. Among the living units which make up a frog, there is much more division of labour than there is among those of Hydra. An excised representative sample of Hydra will reproduce the whole animal, but this is not true of the frog. The structural result of this physiological division of labour is HOMOLOGOUS ORGANS 43 differentiation. The animal, or part of it, becomes more complex, more heterogeneous. If we contrast a bird and a sponge, another great fact in regard to the evolution of organs is illustrated. The bird is more of a unity than a sponge ; its parts are more closely knit together and more adequately subordinated to the life of the whole. This kind of progress is called integration. Differentiation involves the acquisition of new parts and powers, these are consolidated and harmonised as the animal becomes more integrated. Correlation of organs. — It is of the very nature of an organism that its parts should be mutually dependent. The organs are all partners in the business of life, and if one member changes, others also are affected. This is especially true of certain organs which have developed and evolved together, and are knit by close physiological bonds. Thus the circulatory and respiratory systems, the muscular and the skeletal systems, the brain and the sense organs, are very closely linked, and they are said to be correlated. A variation, for better or worse, in one system often brings about a correlated variation in another, though we cannot always trace the physiological connection. Homologous organs. — Organs which arise from the same primitive layer of the embryo (see Chapter IV.) have some- thing in common. But when a number of organs arise in the same way, from the same embryonic material, and are at first fashioned on the same plan, they have still more in common. Nor will this fundamental sameness be affected though the final shape and use of the various organs be very different. We call organs which are thus structurally and developmentally similar, homologous. Thus the nineteen pairs of appendages on a crayfish are all homologous ; the three pairs of " jaws " in an insect are homologous with the insect's legs ; and it is also true that the fore-leg of a frog, the wing of a bird, the flipper of a whale, the arm of a man, are all homologous. The wing of a bird and the arm of man exhibit the same chief bones, blood vessels, muscles, and nerves, and they begin to develop in the same way ; they are homologous hut not analogous. The wing of a bird and the wing of an insect, which resemble one another in being organs of flight, are not the least alike in structure ; 44 THE ELEMENTS OF STRUCTURE they are analogous but not homologous. Yet two organs may be both homologous and analogous^ e.g. the wing of a bird and the wing of a bat, for both are fore-Umbs, and both are organs of flight. Sometimes two organs or two organisms — deeply different in structure — have a marked superficial resemblance, simply because both have arisen in relation to similar conditions of life. Thus a burrow- ing amphibian, a burrowing lizard, and a burrowing snake resemble one another in being limbless, but this ** conver- gence," or " homoplasty," of form does not indicate any relationship between them. Change of function. — Division of labour involves restric- tion of functions in the several parts of an animal, and no higher Metazoa could have arisen if all the cells had remained with the many-sided qualities of Amoebae. Yet we must avoid thinking about organs as if they were necessarily active in one way only. For many organs, e.g. the liver, have several very distinct functions. In addition to the main function of an organ, there are often secondary functions ; thus the wings of an insect may be respiratory as well as locomotor, and part of the food canal of Tunicates and Amphioxus is almost wholly subservient to respiration. Moreover, in organs which are not very highly specialised, it seems as if the component elements retained a consider- able degree of individuality, so that in course of time what was a secondary function may become the primary one. Thus Dohrn, who especially emphasised this idea of function change, says : " Every function is the resultant of several components, of which one is the chief or primary function, while the others are subsidiary or secondary. The diminution of the chief function and the accession of a secondary function changes the total function ; thesecondary function becomes gradually the chief one ; the result is the modification of the organ." The contraction of a muscle is always accompanied by electric changes, and in the electric organs of fishes we see the electric changes in the modified muscular tissue composing the organs becoming more important than the contractility. The structure known as the allantois is an unimportant bladder in the frog, in Birds and Reptiles it forms a foetal membrane (chiefly respiratory) around the embryo, and in most Mammals it forms part of SUBSTITUTION OF ORGANS 45 the placenta which effects vital connection between off- spring and mother. Substitution of organs. — The idea of several changes of function in the evolution of an organ, suggests another of not less importance which has been emphasised by Kleinen- berg. An illustration will explain it. In the early stages of all vertebrate embryos, the supporting axial skeleton is the notochord — a rod developed along the dorsal wall of the gut. From Fishes onwards, this embryonic axis is gradually replaced in development by the vertebral column or backbone ; the notochord does not become the back- bone, but is replaced by it. It is a temporary structure, around which the vertebral column is constructed, as a tall chimney may be built around an internal scaffolding of wood. Yet it remains as the sole axial skeleton in Amphioxus, persists in great part in hag and lamprey, but becomes less and less persistent in Fishes and higher Vertebrates, as its substitute, the backbone, develops more perfectly. Now, what is the relation between the notochord and its substitute the backbone, seeing that the former does not become the latter ? Kleinenberg's suggestion is that the notochord supplies the stimulus, the necessary condi- tion, for the formation of the backbone. Of course we require to know more about the way in which an old- fashioned structure may stimulate the growth of its future substitute, but the general idea of one organ leading on to another is suggestive. It is consistent with our general conception of development — that each stage supplies the necessary stimulus for the next step ; it also helps us to understand more clearly how new structures, too incipient to be of use, may persist, and why old structures should linger though they have only a transitory importance. Rudimentary organs. — In many animals there are struc- tures which attain no complete development, which are rudimentary in comparison with those of related forms, and seem retrogressive when compared with their promise in embryonic life. But it is necessary to distinguish various kinds of rudimentary structures, (a) As a pathological variation, probably due to some germinal defect, or to the insufficient nutrition of the embryo, the heart of a mammal is sometimes incompletely formed, Other organs may be 46 THE ELEMENTS OF STRUCTURE similarly spoilt in the making. They illustrate arrested development, (b) Some animals lose, in the course of their life, many of the prominent characteristics of their larval Hfe ; thus parasitic crustaceans at first free-living, and sessile sea-squirts at first free-swimming, always undergo degenera- tion, which can be seen in each lifetime, {c) But the little kiwi of New Zealand, with mere apologies for wings, and many cave fishes and cave crustaceans with slight hints of eyes, illustrate degeneration, which has taken such a hold of the animals that the young stages also are degener- ate. The retrogression cannot be seen in each lifetime, evident as it is when we compare these degenerate forms with probable ancestors, (d) But among '* rudimentary organs " we also include structures somewhat different, e.g. the gill- clefts which persist in embryonic reptiles, birds, and mammals, though most of them serve no obvious purpose, or the embryonic teeth of whalebone whales. These are " vestigial structures,^' traces of ancestral history, and in- telligible on no other theory. The gill-clefts are used for respiration in all vertebrates below reptiles ; the ancestors of whalebone whales doubtless had functional teeth. Classification of organs. — We may arrange the various parts of the body physiologically, according to their share in the life. Thus some parts liave most to do with the external relations of the animals ; such as locomotor, prehensile, food-receiving, protective, aggressive, and copulatory organs. Of internal parts, the skeletal structures are passive ; the nervous, muscular, and glandular parts are active. The repro- ductive organs are distinct from all the rest. They are conveniently called " gonads," which is a better term than reproductive glands. For by a gland we mean an organ which secretes, whose cells produce and liberate some definite chemical substance, such as a digestive ferment ; whereas the gonads are organs where there is periodic multi- plication of certain cells, kept apart from the specialisation character- istic of most of the " body cells " or " somatic " cells. It is true, however, that an accessory glandular function is often associated with the gonads. Another classification of organs is embryological, i.e. according to the embryonic layer from which the various parts arise. Thus the outer layer of the embryo (the ectoderm or epiblast) forms in the adult — (i) the outer skin or epidermis ; (2) the nervous system ; (3) much at least of the sense organs : the inner layer of the embryo (the endoderm or hypoblast) forms at least an important part (the " mid-gut") of the food canal, and the basis of outgrowths (lungs, liver, pancreas, etc.) which may arise therefrom, and also the notochord of Vertebrates : the middle layer of the embryo (the mesoderm or mesoblast) forms skeleton, connective swathings, muscle, lining of body-cavity, etc. TISSUES 47 III. Tissues. — Zoological anatomists, of whom Cuvier may be taken as a type, analyse animals into their com- ponent organs, and discover the homologies between one animal and another. But as early as 1801, Bichat had published his Anatomie generate, in which he carried the analysis further, showing that the organs were composed of tissues, contractile, nervous, glandular, etc. In 1838-39, Schwann and Schleiden formulated the " cell theory," in which was stated the result of yet deeper analysis — that all organisms have a cellular structure and origin. The simplest animals (Protozoa) are typically single cells or unit masses of living matter ; as such all animals begin ; but all, except the simplest, consist of hundreds of these cells united into more or less homogeneous companies (tissues), which may be compacted, as we have seen, into organs. If we think of the organism as a great city of cells, the tissues represent streets (Hke some of those in Leipzig), in each of which some one kind of function or industry predominates. The student should read the introductory chapters in one of the numerous works on histology, so as to gain a general idea of the character of the different tissues. There are four great kinds — epithelial, connective, muscular, and nervous. (a) Epithelial tissue is illustrated by the external layer of the skin (epidermis), the internal (endothehal) lining of the food canal and its outgrowths, the lining of the body cavity, etc. ; by the early arrange- ments of cells in all embryos ; and by the simplest Metazoa, such as Hydra, whose tubular body is formed by two layers of epithehum. Embryologically and historically, epithelium is the most primitive kind of tissue. It may be single layered or stratified ; its cells may be columnar, scale-like, or otherwise. The cells may be close together, or separated by intercellular spaces, and they are' often connected by bridges of living matter. Nor are the functions of epithelium less diverse than its forms, for it may be ciliated (effecting locomotion, food-wafting, etc.), or sensitive (and as such forming sense organs), or glandular (liberating certain products or even the whole contents of its cells), or pigmented (and thus associated with respiration, excretion, and protection), or covered externally with "'a sweated-off cuticle, susceptible of many modifications (especially of protective value). (b) Connective tissue. — This term includes too many different kinds of things to mean much. It represents a sort of histological lumber- room. The embryologists help us a little, for they have shown that almost all forms of connective tissue are derived from the mesoderm or middle layer of the embryo, As this mesoderm usually arises in the form of 48 THE ELEMENTS OF STRUCTURE outgrowths from the gut, or from (mesenchyme") cells liberated at an early stage from either (?) of the two other layers of the embryo (ectoderm or endoderm), we may say that connective tissue is primarily derived from epithelium. The general function of " connective tissue " is to enswathe, to bind, and to support, but the forms assumed are very various. The cells may be without any intercellular "mortar" or matrix. They may be laden with fat or with pigment. In other cases the cells of the connective tissue lie in a matrix, which they secrete, or into which they in part die away. Sometimes the matrix becomes secondarily invaded by cells. The connective cells are very often irregular in out- line, and give off, in most cases, fine processes, which traverse the matrix as a network. They may secrete long fibres, as in the various kinds of fibrous tissue. The fibrous tissue of tendons and the different kinds of gristle or cartilage illustrate connective tissue with much matrix. Cartilage is sometimes hardened by the deposition of lime salts in its substance, and then has a slight resemblance to another kind of " connective tissue" — bone. But bone, which is restricted to Verte- brate animals, is quite different from the cartilage which it often succeeds and replaces. It is made by strands or layers of special bone-forming cells C, Cilia; B., basal corpuscle at tlic root of (osteoblasts), which mav rest •each cilium; -F-i, fi"^ intracellular hbnls cartilage foundation, or corresponding to the cilia ; A ., nucleus « .« ^ f, with chromosomes darkly stained; (Y., may be quite mdepondent. general cytoplasm of the cell. These osteoblasts form the bone matrix, and some of them are involved in it, and become the permanent bone cells. These have numerous radiating branches, and are arranged in concentric layers, usually around a cavity or a blood vessel. (There are no blood vessels in cartilage.) The matrix becomes very rich in lime salts (especially phosphate) ; and the cartilage foundation, if there was one, is quite destroyed by the new formation. Here we may also note two important fluid tissues, the floating corpuscles or cells of Fig. 22. — Three ciliated cells. MUSCULAR TISSUE 49 the blood, and those of the body cavity or " perivisceral " fluid which is often abundant and important in backboneless animals. (c) Muscular tissue. — The single-celled Amoeba moves by flowing out on one side and drawing in its substance on another. It is diffusely contractile, and it has also sensitive, digestive, and other functions. In Hydra and some other Coelentera the bases of some of the epithelial cells which form the outer and inner layers are prolonged into contractile roots. Here, then, we have cells of which a special part discharges a contractile or muscular func- tion, while the other parts retain other powers. In other Coelentera the muscular cells are still directly connected with the epithelium, but become more and more exclusively con- tractile. In all other animals the muscular tissue is derived from the mesoderm, which, as we have already mentioned, is not dis- tinctly present in Coelentera. In the majority, the muscle-cells arise on the walls of the body cavity, and their origin may often at least be described as epithelial. But in other cases the muscles arise from those wandering " mesenchyme " cells to which we have already referred. Smooth or unstriped muscle fibres are elongated contractile cells, externally homo- geneous in appearance. They are especially abundant in sluggish animals, e.g. Molluscs, and occur in the walls of the gut, bladder, and blood vessels of Vertebrates. They are less perfectly differentiated than striped muscle fibres, and usually contract more slowly. A striped muscle fibre is a cell the greater part of which is modified into a set of parallel longitudinal fibrils, with alternating " clear and dark " transverse stripes. A residue of unmodified cell substance, with a nucleus or with many, is often to be observed on the side of the fibre, and a slight sheath or sarcolemma forms the " cell wall." Many muscle fibres closely combined, and wrapped in a sheath of connective tissue, form a muscle, which, as every one knows, can contract with extreme rapidity when stimulated by a nervous impulse. (d) Nervous tissue. — Beginning again with the Amoeba, we recognise that it is diffusely sensitive, and that a stimulus can pass from one part of the cell to another. In some Coelentera a few of the external cells seem to combine contractile and nervous functions. Therefore they are sometimes called " neuro-muscular." Fig. 23. — A unstriped smooth or muscle-cell. slowly contracting. A'^., Nucleus ; FL., longitudinal intracellular fibrillation. 50 THE ELEMENTS OF STRUCTURE But in Hydra there are superficial sensory cells, whose basal pro- l(jngations are connected either directly with contractile cells, or with deeper ganglion-cells, some of which give off motor processes to the contractile cells. In sea-anemones and some other Coelentera there is a more sharply defined division of labour. Super- ficial sensory cells are connected with subjacent nerve- or ganglion- cells, from which fibres pass to the contractile elements. In higher animals the sensory cells are mostly integrated into sense organs, the ganglionic cells into ganglia, while the delicate fibres which form the connections between sensory cells and gang- lionic cells, and between the latter and muscles, are compacted to form well-developed nerves. So far as we know, nervous tissue always arises from the outer or ectodermic layer of the embryo, as we would expect from the fact that this is the layer which, in the course of history, has been most directly subjected to external stimulus. I-et us consider first the gang- lionic cells which receive stimuli and shunt them, which regulate the whole life of the organism, and are the physical conditions of " spon- taneous " activity and intelligence. They are of very varied shape, but consist always of a cell-body which gives off one or more processes. One of these processes is long, branches very sparingly, and is known as the axis-cylinder. There are usually present other processes which ramify like the branches of a tree and are called dendrites. The cell-body contains a nucleus, distinct granules, and a network of fine fibrils. The nervous system is built up of such "neurones." In the ganglia they are supported and held apart by much-branched neuroglia cells. In all but a few of the simplest Metazoa, the nerve fibres (axis- cylinders) are surrounded by a sheath called the neurilemma, said to be formed by adjacent connective tissue. Several nerve fibres may com- bine to form a nerve, but each still remains ensheathed in its neuri- 5TR Fig. 24. — A piece of striped muscle fibre with its nerve-endings. STR., Striations of tlie muscle fibre ; N., nuclei of the muscle fibre ; M.N., a motor nerve giving off motor nerve fibres (M.F.), which lead to branching motor-endings (M.A.). S.A. is a sensory nerve-ending, from which impulses are carried by sensory fibres (S.F.) to a sensory nerve. CELLS 51 lemma, while fibrous sheaths bind the nerve fibres together. In Verte- brate animals each nerve fibre usually has in addition a medullary sheath. But even in the higher Vertebrates, " non-meduUated " or simply contoured nerve fibres are found in the sympathetic and olfactory nerves, and this simpler type alone occurs in hag, lamprey, and lancelet, as well as in all the Invertebrates with distinct nerves. A nerve fibre contains numerous fibrils Uke those seen within a ganglion cell. These are regarded by some as the essential elements in conducting stimuli, while others maintain that the essential part is the less compact, sometimes well-nigh fluid stuff between the fibrils, or that the fibrils are but the walls of tubes within which the essentially nervous stuff lies. The nerve fibres arise as prolongations of the ganglion cells, which extend themselves in the embryo like Amoebae sending out pseudopodia. IV. Cells. — In discussing tissues, it was necessary to refer to the component cells. Let us now consider the chief characteristics of these elements. A cell is a unit mass or area of living matter usually with a nucleus. Most of the simplest animals and plants (Protozoa and Protophyta) are single cells ; eggs and male elements are single cells ; in multicellular organisms the cells are combined into tissues and organs. Most cells are too small to be distinguished except through lenses ; many Protozoa, e.g. large Amoebae, are just visible to our unaided eyes ; the chalk-forming Fora- minifera are single cells, whose shells are often as large as pin-heads, and some of the extinct kinds were as big as half-crowns (see Fig. 17) ; the bast cells of plants may extend for several inches ; the largest animal cells are eggs distended with yolk. The typical and primitive form of cell is a sphere — a shape naturally assumed by a complex coherent substance situated in a medium different from itself. Most egg-cells and many Protozoa retain this primitive form, but the internal and external conditions of life (such as nutrition and pressure) often evolve other shapes — oval, rectangular, flattened, thread-like, stellate, and so on. As to the structure of a cell, we may distinguish (see Fig- 25)— (a) The general cell substance or cytoplasm, which con- sists partly of genuinely living stufi^ or protoplasm, and partly of complex materials not really living (metaplasm) ; 52 THE ELEMENTS OF STRUCTURE (b) A Specialised nucleus, with a complex structure, and important functions ; (c) One or more specialised bodies called central corpuscles or centrosomes, which seem to be centres of activity during cell division ; {d) A cell wall, which occurs in very varied form, or may be entirely absent. (a) As to the cell substance, it appears in living cells to be Fig. 25. — Diagram of cell structure. — After Wilson. PL., Plastids in cytoplasm ; CC, centrosomes in centrosphere ; n., nucleolus ; N., nucleus ; CHR., chromosomes ; CT., general cytoplasm ; v., vacuole ; GR., granules. clear, colourless, structureless, and more or less fluid. There are great variations in viscosity from cell to cell, from time to time, and even from place to place within a single cell at one instant. In cells " fixed " and prepared for microscopic study the cytoplasm has an artificial reticular or fibrillar structure. The cytoplasm often contains numerous inclusions. Granules, watery droplets or vacuoles, and oily globules are present in varying numbers ; they are regarded as non- NUCLEUS 53 living aggregates of material — stores of nutritive material, or products of the cell's activity, either useful or useless. The scattered thread-like or rod-like mitochondria and the knot-like Golgi apparatus are more often supposed to form part of the living protoplasm. The first-named at least may be seen in living cells, but both are destroyed by the usual methods of fixation, and require special demonstra- tion. {b) As to the nucleus, one at least is present in almost every cell. It used to be said that some very simple animals, which Haeckel called Monera, had no nuclei, but in many cases the nuclei have now been demonstrated. In other cases, e.g. some Infusorians, the nuclear material seems to be diffused in the cell substance. The red blood cells of Mammals seem to be distinctly nucleated in their early stages, though there is no nucleus in those which are full grown. The nucleus is a very important part of the cell, but it is not yet possible to define precisely what its importance is. In fertilisation an essential process is the union of the nucleus of the spermatozoon or male cell with the nucleus of the ovum or female cell (Fig. 27). In cell division the nucleus certainly plays an essential part. Cells bereft of their nuclei die, or live for a while a crippled life. Accord- ing to some, the nucleus is important in connection with the nutrition of the cell ; according to others, it is of special importance in connection with the respiration of the cell. It is certain that there are complex actions and reactions between the living matter of the nucleus and that of the cytoplasm. Cytoplasm and nucleoplasm form a " cell firm," potent in their co-operation. In many cells it has been show^n that fragments or extensions of the nucleus pass into the cytoplasm, forming what is called a " chromidial apparatus," which seems to be of much functional im- portance. The nucleus often lies within a little nest in the midst of the cell substance, but it may shift its position from one part of the cell to another. It has a definite margin, but this may be lost, e.g. before cell division begins. Inter- nally, the living resting nucleus appears to be fluid and quite homogeneous ; but if injured mechanically or 54 THE ELEMENTS OF STRUCTURE chemically (in fixation) it coagulates readily in a pattern which is anything but homogeneous (see Fig. 26). Twisted strands or tubes of " linin " bear a more stainable material called " chromatin," and when the cell is preparing to divide the strands assume the form of a definite number of separable rods or loops or granules, the " chromosomes." The number of chromosomes is in general constant for each species of animals and plants. Surrounding the linin and chromatin is the nuclear sap. Sometimes a linin thread shows a row of minute chromatin bodies (microsotnata), like jewel-stones embedded on a belt. Weismann suggested that the chromosomes or idants of the germ-cells are the vehicles of the heritable qualities or of some of them ; and this view is generally accepted. They carry the hereditary " factors " or " genes," apparently in a linear arrangement. Many nuclei also contain little round bodies or nucleoli, or sometimes a single nucleolus. The term is applied somewhat vaguely to little aggrega- tions of chromatin, and more properly to vacuole-like bodies, in which some believe that the waste products of the nucleus are collected. {c) As to the centrosomes, it may be note protoplasmic notcd that whcu an animal cell divides, SJJedb^'fixhit'S these bodies play an important part. The chromatin elements of the nucleus are divided, and separate to form the two daughter nuclei. In this separation extremely fine " archoplasmic " threads appear to pass from the centrosomes to the chromosomes. The centrosomes are therefore regarded as " division organs," or as " dynamic centres." They also occur, in most cases singly, in resting cells, and it seems likely that they are present in most animal cells, at least in those which retain the power of division. {d) As to the cell wall, it seemed of much moment to the earlier histologists, who often spoke of cells as little bags or boxes. It is, however, the least important part of the cell. In plant cells there is usually a very distinct wall, consisting of cellulose. This is a product, not a Fig. 26. — Structure of the cell. — After Carnoy. N ., Nucleus with chromatin coil CELL WALL 55 Fig. 27. — Fertilised ovum of Ascaris. — After Boveri. chr., Chromosomes, two from ovum nucleus and two from sperm nucleus ; cs., centro- some from which " archo- plasmic " threads i^adiate, partly to the chromosomes. part, of the protoplasm, though some protoplasm may be intimately associated with it as long as its growth con- tinues. In animal cells there is rarely a very distinct wall chemically distinguishable from the living matter itself. But the margin is often different from the interior, and a slight wall may be formed by a superficial physical alteration of the cell substance, com- parable to the formation of a skin on cooling porridge. In other cases, especially in cells which are not very active, such as ova and encysted Protozoa, a more definite sheath is formed around the cell substance. Again, animal cells may secrete a superficial " cuticle," e.g. the chitin formed by the ectoderm cells in Insects, Crustaceans, and other Arthropods. The " plasma membrane," which fornis the outer boundary of the protoplasm, is invisibly thin ; its exist- ence and properties are deduced indirectly from experi- ments on its impermeability to various substances. The " micro-dissection " experiments of Chambers have made it more real to us in the last few years, and have emphasised its great im- portance in the life of the cell. In animals, as well as in plants, adjacent cells are often linked by intercellular bridges of living matter, which may be paths for the passage of materials or of dis- turbances from cell to cell. In many cases,- e.g. of gelatinous tissue, a matrix arises outside of and between the cells, as an exo- plasmic product. In regard to cell division, the most important facts are the following : — There is a striking similarity in most cases, and the nucleus plays an essential part in the process. The chr cs Fig. 28. — Diagram of cell division. — After Boveri. chr., Chromosomes forming an equatorial plate ; cs., centro- some. 56 THE ELEMENTS OF STRUCTURE dividing nucleus usually passes through a series of complex changes known as karyokinesis or mitosis, and these are much the same everywhere, though different kinds of cells have their specific peculiarities. Occasionally, however, both in Protozoa and Metazoa, the nucleus divides by simple constriction (direct or amitotic division). This is a quicker process than the other, and occurs especially when there is rapid growth or frequent replacement of cells. Another departure from the ordinary scheme is seen when the nucleus shows a multiple division, while the cell remains undivided. This occurs normally in some marrow cells. The eventful changes of karyokinesis are as follows : — (a) The resting stage of the nucleus shows a network or complete coil of filaments (chromatin elements) (Fig. 29). {[■>) First stage. — As division begins, the membrane separating the nucleus from the cell substance disappears, and the chromatin elements are seen as a tangled or broken coil (Fig. 20, I). (c) Astroid stage.— The chromatin elements bend into looped pieces (or chromosomes), which are disposed in a star, lying flat at the equator of the cell, the free ends of the U-shaped loops being directed outwards. Meanwhile a centrosome has appeared and divided into two separating halves, between which a spindle of fine achromatin threads is formed. This seems to form (at least part of) what is called the nuclear spindle. The centrosomes separate until one lies at each pole of the cell, surrounded by radiating " archoplasmic " threads which become attached to the chromosomes (Fig. 29, 2). {d) Division and separation of the loops. — Each of the loops which make up the star divides longitudinally into two, and each half separates from its neighbour. They lie at first near the equator of the cell, but they are apparently drawn, or driven, to the opposite poles (Fig. 29, 2-4). (e) Diastroid. — The single star thus forms two daughter stars, which separate farther and farther from one another towards the opposite poles of the cell, remaining connected, how- ever, by delicate threads (Fig. 29, 3-5). (/) Each daughter star is reconstituted into a coil or network for each daughter cell, for the cell substance has been con- stricted meanwhile at right angles to the transverse axis of the spindle. The halves separate in the case of Protozoa, but in most other cases, e.g. growing embryos, they remain •-. adjacent, with a slight wall between them (Fig. 29, 6). (g) Each daughter nucleus then passes into the normal resting phase. The spindle disappears, and the centrosomes may also vanish. KARYOKINESIS 57 The essential fact is the exact partition of the nuclear material between the two daughter cells. It may be added that these various complexities of structure can be seen in living cells as well as in fixed and stained material. Flemming gives the following summary of karyokinesis : — Mother Nuclei's Daughter Nucleus (progressive changes). (regressive changes). a Resting stage. Resting stage. g ^ b Coil. Coil. / c Astroid. Diastroid. e i d Division of Astroid and its loops ^ (Prophases) (Aletakinesis) (Anaphases). c c C'C Fig. 29. — Karyokinesis. — After Flemming. 1. Coil stage of nucleus ; ex., central corpuscle. 2. Division of chromatin elements into U-shaped loops, and longitudinal splitting of these chromosomes (astroid stage). 3. 4. Recession of chromosomes from the equator of the cell (diastroid). 5. Nuclear spindle, with chromatin elements at each pole, and achromatin threads between. 6. Division of the cell completed. Besides the ordinary indirect divisipn just described, the net result of which is that each of the two daughter cells gets the normal number of chromosomes, a precise half of each of the chromosomes in the original cell, there is another kind of cell division (meiotic or reducing division) which occurs only in the maturation of the ovum and spermatozoon, and has for its net result the reduction of the number of chromosomes to a half of the normal number. 58 THE ELEMENTS OF STRUCTURE We are far from being able to give even an approximate account of the " mechanism " of cell division. The whole process is vital, and cannot, at present at least, be re- described in terms of matter and motion. On the other hand, Leuckart, Spencer, and Alexander James have given a general rationale of cell division. Why do not cells grow much larger } why do they almost always divide at a definite limit of growth ? The answer is as follows : — Suppose a young cell has doubled its original volume, that means that there is twice as much living matter to be kept alive. But the living matter is fed, aerated, purified through its surface, which, in growing spherical cells, for instance, only increases as the square of the radius, while the mass increases as the cube. The surface growth always lags behind the increase of mass. Therefore, when the cell has, let us say, quadrupled its original volume, but by no means quadrupled its surface, difficulties set in, waste begins to gain on repair, anabolism loses some of its ascendancy over katabolism. At the limit of growth the cell divides, halving its mass and gaining new surface. It is true that the surface may be increased by out- flowing processes, just as that of leaves by many lobes ; and division may occur before the limit of growth is reached ; but, as a general rationale, applicable to organs and bodies as well as to cells, the suggestion above outUned is very helpful. It is supported by an experiment due to Hart- mann, who kept an Amoeba alive and healthy for over four months, without any division, by amputating small portions of the cytoplasm each day so that the size remained constant. Amoebae which were not operated on divided every second day. The ratio of the amount of nuclear material in the cell to the amount of cytoplasmic material seems also to have a determining influence upon cell division (R. Hertwig). Protoplasm. — Morphological as well as physiological analysis passes from the organism as a whole to its organs, thence to the tissues, thence to the cells, and finally to the protoplasm itself. But although we may define protoplasm as genuinely living matter — as " the physical basis of life " — we cannot definitely say how much or what part of an Amoeba, or an ovum, or any other cell, is really protoplasm. PROTOPLASM 5 9 We are able to make negative statements, e.g. the yolk of an egg is not protoplasm, but we cannot make positive statements, or say. This is protoplasm, and nought else. Thus what is spoken of as the structure of protoplasm is really the structure of the cytoplasm. Sections of fixed and stained cells often show a considerable com- plexity of structure, and various appearances have been often described. Thus some, e.g. Frommann, describe a network or reticulum, with less stable material in the meshes ; others, e.g. Flemming, describe a manifold coil of fibrils ; and others, e.g. Biitschli, describe a foam- like or vacuolar structure. Hardy has imitated these structures by treating perfectly homogeneous colloidal solutions, of egg-white, for example, with various fixatives. Professor BUtschli's belief that the cytoplasm has a vacuolar structure is corroborated by his interesting experiments on microscopic foams. Finely powdered potassium carbonate is mixed with olive oil which has been previously heated to a temperature of 5o°-6o'' C, an acid from the oil splits up the potassium carbonate, liberates carbon dioxide, and forms an extremely fine emulsion. Drops of this show a structure not unlike that of cytoplasm, exhibit movements and streamings not unlike those of Amoebse, and are, in short, mimic cells. Just as a working model may help us to understand the circulation, so these oil-emulsion drops may help us to understand the living cell, by bringing the strictly vital phenomena into greater prominence. More recent work, especially with the ultra-microscope, points to the conclusion that the reticular, fibrillar, and other complexities are, in the main, post-mortem effects. There are definite formed bodies, such as mitochondria and various plastids, in many cells, and there is often a deposition of less labile material by the ever-changing protoplasm, but the important fact is that protoplasm is a heterogeneous mixture in a colloid state. CHAPTER IV THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS I. Reproduction In the higher animals the beginnings of individual hfe are hidden, within the womb in Mammals, within the egg-shell in Birds. It is natural, therefore, that early preoccupation with those higher forms should have hindered the recogni- tion of what seems to us so evident, that almost every animal arises from an egg-cell or ovum which has been fertilised by a male cell or spermatozoon. The exceptions to this fact are those organisms w^hich multiply by buds or detached overgrowths, and those which arise from an egg- cell which requires no fertilisation. Thus Hydra may form a separable bud, much as a rose-bush sends out a sucker ; thus drone-bees " have a mother, but no father," for they arise from parthenogenetic eggs which are not fertilised. Sexual reproduction. — There is apt to be a lack of clearness in regard to sexual reproduction, because the pro- cess which we describe by that phrase is a complex result of evolution. It involves two distinct facts— {a) the liberation of special germ cells from which new individuals arise ; {b) the union or amphimixis of two different kinds of germ cells, ova and spermatozoa, which come to nothing unless they unite. Furthermore, these dimorphic reproductive cells are produced by two different kinds of individuals (females and males), or from different organs of one individual, or at different times within the same organ (hermaphroditism). It is conceivable that organisms might have gone on multiplying asexually, by detaching overgrown portions of 60 GERM CELLS 6 1 themselves which had sufficient vitahty to develop into complete forms. But a more economical method is the liberation of special germ cells, in which the qualities of the organism are inherent. This is the primary characteristic of sexual, as opposed to asexual, multiplication. It is also conceivable that organisms might have re- mained approximately like one another in constitution, and at all times very nearly the same, and that they might have liberated similar germ cells capable of immediate develop- ment. Such a race would have illustrated the one char- acteristic of sexual reproduction, the liberation of special germ cells ; but it would have been without that other characteristic of sexual reproduction — the amphimixis or fertilisation of dimorphic germ cells, usually produced by different organs in one individual or by distinct male and female individuals. Liberation of special germ cells. — One must think of this as an economical improvement on the method of start- ing a new life by asexual overgrowth or by the liberation of buds. Asexual reproduction, as Spencer and Haeckel point out, is a mode of growth in which the bud, or whatever it is, becomes distinct or discontinuous from the parent. The buds of a sponge, of a coral, of a sea-mat, or of many Tunicates, remain attached to the parent. If there be a keen struggle for subsistence, this may be disadvantageous ; but in som.e cases, doubtless, the colonial life which results is a source of strength. In the case of Hydra, however, the buds are set adrift ; the same is true of not a few worms. This liberation of buds takes us nearer the sexual process of liberating special germ cells. But unless the organism is in very favourable nutritive conditions, in which over- growth is natural, the liberation of buds is an expensive way of continuing the life of a species. Not only so, but we can hardly think of budding even as a possibility in very complex organisms, like snails or bitds, in which there is much division of labour. Moreover, the peculiarity of true germ cells is that they do not share in building up the " body," and that they retain an organisation continuous in quality with the original germ cell from which the parent arose ; they are thus not very liable to be tainted by the mishaps which may befall the " body " which bears them. 62 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS And, finally, in the mixture of two units of living matter which have had different histories, an opportunity for new permutations and combinations, in other words, for variation, is supplied. Thus it is not surprising to find that the asexual method of liberating buds has been re- placed in most animals by the more economical and ad- vantageous process of sexual reproduction. Summary of Modes of Reproduction A. In Single-celled Animals (Protozoa) (i) The almost mechanical rupture of an amoeboid cell, which has become too large for physiological equilibrium. (2) The discharge of numerous superficial buds at once {e.g. Arcella and Pelomyxa). (3) The formation of one bud at a time (very common). (4) The ordinary division into two daughter cells at the limit of growth. (5) Repeated divisions within limited time and within limited space (a cyst). This results in what is called spore-formation {e.g. in Sporozoa). B. In Many-celled Animals (Metazoa) {Asexual) {a) The separation of a clump of body cells, e.g. from the surface of some Sponges. (A crude form of budding.) {b) The formation of definite buds which may or may not be set free, (c) Various forms of fission and fragmentation. {Sexual) The liberation of special reproductive or germ cells, which have not taken part in the formation of the body, and which retain the essential qualities of the original germ cell from which the parent arose. These special germ cells — the ova and sperma- tozoa — are normally united in fertilisation, but some animals have (parthenogenetic) ova which develop without being fertilised. Evolution of sex. — A further problem is to account for the two facts — {a) that most animals are either males or females, the former liberating actively motile male elements or spermatozoa, the latter forming and usually liberating EVOLUTION OF SEX 63 more passive egg-cells or ova ; and {b) that these two different kinds of reproductive cells usually come to nothing unless they combine. The problem is partly solved by a clear statement of the facts. Let us begin with those interesting organisms which are on the border line between Protozoa and Metazoa, the colonial Infusorians, of which Volvox is a type. The adults are balls of cells, and the component units are con- nected by protoplasmic bridges. From such a ball of cells reproductive units are sometimes set adrift, and these divide to form other individuals without more ado. In other con- ditions, however, when nutrition is checked, a less direct mode of reproduction occurs. Some of the cells become large, well-fed elements, or ova ; others, less successful, divide into many minute units or spermatozoa. The large cells are fertilised by the small. Here we see the formation of dimorphic reproductive cells in different parts of the same organism. But we may also find Volvox balls in which only ova are being made, and others with only spermatozoa. The former seem to be more vegetative and nutritive than the latter ; we call them female and male organisms respectively ; we are at the foundation of the differences between the two sexes. All through the animal series, from active Infusorians and passive Gregarines to feverish Birds and more sluggish Reptiles, we read antitheses between activity and passivity, between lavish expenditure of energy and a habit of storing. The ratio between disruptive {katabolic) processes and con- structive {anabolic) processes in the protoplasmic meta- bolism varies from type to type. It may be that the contrast between the sexes is another expression of this fundamental alternative of variation. Stages in the history of fertilisation. — While it is not difficult to see the advantage of fertilisation as a process which helps to sustain the standard or average of a species and as a source of new variations, we can at present do little more than indicate various forms in which the process occurs. (a) Formation of Plasmodia, the flowing together of numerous feeble cells, as seen in the life-history of those very simple Protozoa called Proteomyxa, e.g. Protomyxa, and Mycetozoa, e.g. flowers of tan {/Ethalium septicum). {b) Multiple conjugation, in which more than two cells unite and fuse 64 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS together temporarily, as in some Sporozoa and in the sun- animalcule (A ctinosplun-iiim). (c) Ordinary conjugation, in which two similar cells unite, with fusion of their nuclei, observed in Sporozoa, Heliozoa, Flagel- lates, and Rhizopods. In ciliated Infusorians, the conjugation may be merely a temporary union, during which nuclear elements are interchanged. (d) Dimorphic conjugation, in which two cells different from one another fuse into one, a process well illustrated in Vorticella and related Infusorians, where a small, active, free-swimming (we may say, male) cell unites with a fixed individual of normal size, which may fairly be called female (see Fig. 53). (e) Fertilisation, in which a spermatozoon liberated from a Metazoon unites intimately with an ovum, usually liberated from another individual of the same species. Divergent modes of sexual reproduction. — (a) Herm- aphroditism is the combination of male and female sexual functions in varying degrees within one organism. It may be demonstrable in early life only, and disappear as male- ness or femaleness predominates in the adult. It may occur as a casualty or as a reversion ; or it may be normal in the adult, e.g. in some Sponges and Coelentera, in many " worms," such as earthworm and leech, in barnacles and acorn-shells, in one species of oyster, in the snail, and in many other Bivalves and Gastropods, in Tunicates and in the hag-fish. In most cases, though these animals are bisexual, they produce ova at one period and spermatozoa at another (dichogamy). It rarely occurs (e.g. in some parasitic worms) that the ova of a hermaphrodite are fertilised by the sperms of the same animal (autogamy). Certain facts, such as the occurrence of hermaphrodite organs as a transitory stage in the development of the embryos of many unisexual animals (e.g. frog and bird), suggest that hermaphroditism is a primitive condition, and that the unisexual condition of permanent maleness or femaleness is a secondary differentiation. Other facts, such as the hermaphroditism of many parasites, where cross- fertilisation would be difficult, suggest that the bisexual condition may have arisen as a secondary adaptation. It seems likely that there is both primitive and secondary hermaphroditism. (b) Parthenogenesis, as we know it, is a degenerate form of sexual reproduction, in which ova produced by a female ALTERNATION OF GENERATIONS 6s organism develop without being fertilised by male elements. It is well illustrated by Rotifers, in which fertilisation is the exception (in some genera males have never been found) ; by many small Crustaceans whose males are absent for a season ; by Aphides, from among which males may be absent for the summer (or in artificial conditions for several years) without affecting the rapid succession of female generations ; by the production of drones in the bee-hive from eggs which are never fertilised. {c) Alternation of generations. — A fixed asexual hydroid I. ^-^ ^R rr;^ /^r r r^ — ^ Fig. 30. — Diagrammatic expression of alternation of generations. 1. HydromedusaB. ov., Fertilised ovum (ov.) gives rise to an asexual form A, which, by budding, produces sexual form or forms S ; in the case of Hydromedusae, A is represented by hydroid (H), and S by medusoid (M). 2. Liver Fluke. ov., Fertilised ovum (ov.) gives rise to asexual stages (A), which, from special spore-like cells (R), produce eventually the sexual fluke (S). or zoophyte often buds off and liberates sexual medusoids or swimming-bells, whose fertilised ova develop into embryos which become fixed and grow into hydroids (Figs. 90 and 107). This is the simplest illustration of alternation of generations, which may be defined as the alternate occurrence in one life-cycle of two (or more) different forms differently produced (Fig. 30). The liver-fluke (Distomum hepaticum) of the sheep produces eggs which, when fertilised, grow into embryos. Within the latter, certain cells (which might be called 5 66 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Spores) grow into numerous other larvae of a different form. Within these the same process is repeated, and finally the larvae thus produced grow (in certain con- ditions) into sexual flukes (Fig. ii8). In this case, reproduction by special cells, like undifferentiated pre- cocious ova, alternates with reproduction by ordinary fertilised egg-cells. So, too, the vegetative sexless " fern- plant " gives rise to special spore cells, which develop into an inconspicuous bisexual " prothallus," from the fertilised egg-cell of which a " fern-plant " springs. Various kinds of alternation are seen in the life-cycle of the fresh-water sponge, in the stages of the jelly-fish Amelia, in the history of some " worms" and Tunicates. They illustrate a rhythm between asexual and sexual multi- plication, between parthenogenetic and normal sexual reproduction, between vegetative and active life, between a relatively " anabolic " and a relatively " katabolic " preponderance. II. Embryology Egg-cell or ovum.^ — Apart from cases of asexual repro- duction and parthenogenesis, every multicellular animal begins life as an egg-cell with which a male cell or sperma- tozoon has entered into intimate union. The most important characteristic of the reproductive cells, whether male or female, is that they retain the essential qualities of the fertilised ovum from which the parent animal was developed. The ovum has the usual characters of a cell ; its cyto- plasm is a complex colloidal mixture of substances ; its nucleus or germinal vesicle contains the usual chromatin elements ; it has often a store of reserve material or yolk, and a distinct envelope representing a cell wall (Figs. 31 and 37). In Sponges the ova are well-nourished cells in the middle stratum of the body ; in Coelentera they seem to arise in connection with either outer or inner layer (ectoderm or- endoderm) ; in all other animals they arise in connection with the middle layer or mesoderm, usually on an area of the epithelium lining the body cavity. In lower animals THE EGG-CELL 67 they often arise somewhat diffusely ; in higher animals their formation is restricted to distinct regions, and usually to definite organs — the ovaries. The young ovum is often amoeboid, and that of Hydra retains this character for some time (Fig. 89, 2). The ovum grows at the expense of adjacent cells, or by absorbing material which is contributed by special yolk glands or supplied by the vascular fluid of the body. The yolk or nutritive capital may be small in amount, and distributed uniformly in the cell, as in the ova of Mammals, earthworm, starfish, and sponge ; or it may be Fig. 31.— Diagram of ovum, showing diffuse yolk granules. g.v., Germinal vesicle or nucleus ; chr., chromatin elements or chromosomes. more abundant, sinking towards one pole as in the egg of the frog, or accumulated in the centre as in the eggs of Insects and Crustaceans ; or it may be very copious, dwarf- ing the formative protoplasm, as in the eggs of Birds, Reptiles, and most Fishes (Fig. 39). Round the egg there are often sheaths or envelopes of various kinds — (a) made by the ovum itself, and then very delicate (e.g. the vitelline membrane) ; [b) formed by ad- jacent cells {e.g. the follicular envelope) ; or (c) formed by special glands or glandular cells in the walls of the oviducts {e.g. the " shells " of many eggs). The envelope is often 68 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS firm, as in the Fig. 32. — Diagram of a typical sper- matozoon. //., The so-called " head " ; A., acro- some, which first attaches itself to the egg-cell in fertilisa- tion; CHK., chromo- somes, the vehicles of many hereditary factors, if not of all ; Cy., the protoplasm of the head ; M.P., " middle piece " of the spermatozoon, including the ccntro- some (C.) ; T., the locomotor tail, with an axial filament {A.F.) running down the middle. chitinoiis coat around the eggs of many Insects, and in these cases we find a minute aperture (micropyle), or several of them, through which the sperma- tozoon can enter. The hard calcareous shells round the eggs of Birds and Tor- toises, or the mermaid's purse enclosing the egg of a skate, are of course formed after fertilisation. Egg-shells must be dis- tinguished from egg capsules or cocoons, e.g. of the earthworm, in which several eggs are wrapped up together. Male cell or spermatozoon. — This is a much smaller and usually a much more active cell than the ovum. In its minute size, locomotor energy, and persistent vitality, it resembles a flagellate Monad, while the ovum is comparable to an Amoeba or to one of the more encysted Protozoa. A spermatozoon has usually three dis- tinct parts : the essential " head," con- sisting mainly of nucleus, and the mobile " tail," which is often fibrillated, and a small middle portion between head and tail, which is usually the bearer of the centrosome. The spermatozoa of Thread- worms and most Crustaceans are sluggish, and inclined to be amoeboid (Fig. 33 (6, 7))- Both ova and spermatozoa are true cells, and they are complementary, but the spermatozoon has a longer history behind it (Fig. 34). The homologue of the ovum is the mother sperm cell or spermatogonium. This segments as the ovum does, but the cells into which it divides have little coherence. They go apart, and become spermatozoa. There is often a resemblance between the difl^erent ways in which a mother sperm SPERMATOZOA 69 cell divides and the various kinds of segmentation in a fertilised ovum. In most cases the spermatogonium Yic. 33.— Forms of spermatozoa (not drawn to scale). I and 2. Immature and mature spermatozoa of snail ; 3- of bird ; 4. of man (h., head ; m., middle portion ; /., tail) ; 5- 01 sala- mander, with vibratile fringe (/.) ; 6. of Ascaris, slightly amoeboid with cap (c.) ; 7. of crayfish. divides into spermatocytes, which usually divide again mto spermatids or young spermatozoa. YiG, 34. — Diagram of maturation and fertilisation. (From Evolution of Sex.) A. Primitive sex cell, supposed to be amoeboid. B. Unripe ovum ; C. formation of first polar body (i. p.b.) ; D. forma- tion of second polar bodv (2. p.b.). B'. Mother sperm cell ; C the same divided (sperm-morula). D'. Ball of immature spermatozoa ; sp., liberated spermatozoa. E. Process of fertilisation ; F. approach of male and female nuclei within the ovum. Maturation of ovum. — When the egg-cell attains its definite size or limit of growth, it bursts from the ovary or 70 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS from its place of formation, and in favourable conditions meets either within or outside the body with a spermatozoon from another animal. Before the union between ovum and spermatozoon is effected, generally indeed before it has begun, the nucleus or germinal vesicle of the ovum moves to the periphery and divides twice. This division results in the formation and extrusion of two minute cells or polar bodies, which come to nothing, though they may linger for a time in the precincts of the ovum, and may even divide. The second division follows the first without the inter- vention of the " resting stage " which usually succeeds a nuclear division. In most cases the division which forms the first polar body is a reducing or meiotic division, the number of chromosomes being reduced to half the number characteristic of the cells of the body. The extrusion of polar globules and the associated reduction is almost universal in the history of ova, but in some parthenogenetic ova only one polar body is formed, and there is no reduc- tion in the number of chromosomes. In some other cases the parthenogenetic ovum passes through the meiotic phase and forms two polar bodies. The second of these, however, is not liberated, but remains within the ovum and re-uniting with the reduced nucleus restores the normal number of chromosomes. Reducing or Meiotic Division. — In each kind of animal there is a definite number of chromosomes, say n, in each of the body-cells. In the ripe germ-cells, however, there is half the normal number, ", so that when spermato- zoon and ovum unite in fertilisation the normal number is restored. In the history of the germ-cells, therefore, in one way or another, at one stage or another, the number of chromo- somes undergoes reduction to half the normal. In many cases this reduction comes about through a " heterotypic " or meiotic division. We give a condensed account of what happens in a large number of cases. The immature germ-cells, whether oocytes or spermatocytes, show n chromosomes, half of which are of paternal, and half of maternal, origin. At a certain stage in the ripening or inatnration there is a conjugation of the chromosomes in pairs, and the two forming a pair seem to be of maternal and paternal origin. MEIOTIC DIVISION 71 In the reducing, meiotic, or maturation division each daughter-cell gets one or the otlier member of each pair of homologous chromosomes. In the case of the ovum the meiotic division usually occurs in the formation of the first polar body, so that it and the reduced nucleus of the ovum have each " chromosomes. There is no further reduction in 2 the formation of the second polar body, which involves an ordinary equation-division. The first polar body often divides into two. Thus Y^mT- -Vt/. la • • I \l 'V V V,, • • • IPi \f\ Fig. 35. — Oogenesis and Spermatogenesis. — After Boveri. I. and I. A. Primordial germ-cells. II. -IV. and II.A-IV.A. Multiplication of germ-cells (oogonia and sperma- togonia). V. An immature full-grown egg-cell. VI. It gives off the first polar body (P.B.i) by a meiotic division, and the first polar body may divide again (i, 2). VII. The reduced oocyte gives off a second polar body (3) by an equation division, and thus becomes the ripe egg (4). V.A. A spermatogonium which divides by a meiotic division to form two spermatocytes (VI. a). Each of these divides again by an equation division, forming four sperma- tids, which differentiate into spermatozoa (1A-4A). the result is one viable cell (the mature ovum) and three non-viable cells (the polar bodies), each with - chromosomes. In the spermatogenesis or production of spermatozoa the meiotic division is usually the second-last. A " mother-sperm cell " or spermatogonium divides into spermatocytes with n chromosomes, each of these divides into 2 spermatocytes with ^ chromosomes, and these 72 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS again divide into spermatocytes which differentiate into spermatozoa- The result is that from each of the penultimate generation of spermato- cytes there arise four spermatozoa, each with - chromosomes. Thus there is a close parallelism in the maturation process in the two sexes. That the fertilisation of the ovum restores the number to the normal n is obvious. Part of the significance of meiotir division is that it affords oppor- tunity for fresh permutations and combinations of hereditary qualities, for chromosomes are the bearers of at least some of these. It is important to understand that in ordinary mitosis or cell-division, each daughter-cell gets an absolutely similar half of each chromosome of the mother-cell, whereas in meiotic division each daughter-cell gets half of the total number. If we compare the nucleus and its chromosomes to such a common- place thing as a box of matches we may make the difference between the two kinds of division obvious. We might halve the matches by putting half of them into another box (meiotic division) ; or we might take a knife and split each match longitudinally and put one of the sets of halves into another box (ordinary equation division). Fertilisation. — In the seventeenth and eighteenth cen- turies, some naturaUsts, nicknamed " ovists," beHeved that the ovum was all-important, only needing the sperm's awakening touch to begin unfolding the miniature model which it contained. Others, nicknamed " animalculists," were equally confident that the sperm was essential, though it required to be fed by the ovum. Even after it was recognised that both kinds of reproductive elements were essential, many thought that their actual contact was un- necessary, that fertilisation might be effected by an aura seminalis. Though spermatozoa were distinctly seen by Hamm and Leeuwenhoek in 1679, their actual union with ova was not observed till 1843, when Martin Barry detected it in the rabbit. Of the many facts which we now know about fertilisa- tion, the following are the most important : — (i) Apart from the occurrence of parthenogenesis in a few of the lower animals, an ovum begins to divide only after a spermatozoon has united with it. After one sper- matozoon has entered the ovum, the latter ceases to be receptive, and other spermatozoa are excluded. If, as rarely happens, several spermatozoa effect an entrance into the ovum, the result is usually some abnormality. It is said, however, that the entrance of numerous spermatozoa (polyspermy) is frequent in insects and Elasmobranch fishes. FERTILISATION 73 (2) The union of spermatozoon and ovum is very in- timate ; the nucleus of the spermatozoon and the reduced nucleus of the ovum approach one another, combining to form a unified nucleus. (3) The ovum centrosome disappears before fertilisation, but a centrosome is introduced or evoked by the spermato- zoon. It divides into the two which play an important role in the segmentation of the fertilised ovum. (4) When the combined or segmentation nucleus begins the process of development by dividing, each of the two daughter nuclei which result consists partly of material 11 m Fig. 36.— -Fertilisation of egg-cell. — After Fol. I. Shows a minute hillock of protoplasm rising from the ovum towards the approaching spermatozoon. II. Shows how the head of the successful spermatozoon has entered the ovum. III. The tail is nipped off when the head has entered. A pellicle — the fertilisation membrane — is seen around the o\'um. derived from the sperm nucleus, partly of material derived from the ovum nucleus. In other words, the union is orderly as well as intimate, and the subsequent division is so exact, that the qualities marvellously inherent in the sperm nucleus (those of the male parent), and in the ovum nucleus (those of the mother animal), are diffused through- out the body of the offspring, and persfst in its reproductive cells. (5) The spermatozoon may be able to enter the egg only through pre-existing apertures or micropyles, or it may be restricted to a particular region of the egg where the cyto- plasm is not too heavily charged with yolk ; in the simplest cases, as, for example, in annelids, echinoderms, and 74 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS mammals, it may enter at any point. Entrance is effected partly by the boring motion of the spermatozoon, but partly, in some cases at least, by an active engulfing action on the part of the egg ; the whole process is usually com- plete within one minute. In the most studied case, the sea-urchin egg, the first visible change in the egg follows almost immediately : the delicate vitelline membrane becomes detached from the surface of the egg to form the Fig. 37- — Diagram showing relative size of an egg-cell and a sperm -cell (S.). A'., Nucleus ; C, colloidal cytoplasm ; CHR., portions of chromosomes ; G.S., the nucleolus, which used to be called the germinal spot. fertilisation membrane ; the space between this and the egg itself is filled with a clear fluid. There is a striking increase in the permeability of the surface of the ovum during these changes, so that dissolved substances in the cytoplasm may escape altogether and be lost, and the egg is most sensitive to abnormalities in the external medium. Moreover, there is a great increase in the metabolism of the ovum after fertilisation : increased consumption of oxygen, increased evolution of COg, and increased heat production. FERTILISATION 75 (6) Some eggs, e.g. of sea-urchins, can be artificially induced to develop without fertilisation (by being im- mersed for a couple of hours in a mixture of sea-water and solution of magnesium chloride, and by many other means). It seems, therefore, justifiable and useful to distinguish in ordinary fertilisation, (a) the mingling of the hereditary qualities of the two parents, and {b) an Fig. 38. — Fertilisation in A^caris megalocephala. — After Boveri. 1. Spermatozoon (sp.) entering ovum, which contains reduced nucleus (N.), having given off two polar bodies {p.b. i and 2). 2. Sperm nucleus (the upper), and ovum nucleus (A^.). each with two chromosomes, and with centrosomes {c.s.). 3. Centrosomes (c.s.) with " archoplasmic " threads radiating outwards in part to the chromosomes of the two approximated nuclei. 4. Segmentation spindle before first cleavage. exciting or liberating stimulus which induces the ovum to divide. In one interesting case (the thread-worm Rhabditis) the spermatozoon has the latter function, but not the former ; it enters the ovum and stimulates it to divide, but degenerates without fusing with the egg-nucleus. But in more normal cases, it is found that if the fertilised egg is cut in two before the nuclei have united, the half containing the spermatozoon nucleus may divide and 76 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS develop {merogony), but the half containing only the ovum nucleus degenerates. It should be noted that the chromosomes of the spermatozoon do not fuse with the chrom.osomes of the ovum when fertiUsation occurs. They are associated together and divide together in all the cell- • divisions, whether of body-making or of the germ-cell lineage. In some of the divisions of the germ-cell lineage there seems to be an interesting interchange or " crossing over " of pieces of the members of a pair of chromosomes. A special chromosome of the germ-cells seems often to have to do with sex, whether as determiner of maleness or femaleness, or as an index of these two physiological conditions. Segmentation. — The different modes of division ex- hibited by fertilised egg-cells depend in great measure on the quantity and disposition of the passive and nutritive yolk material, which is often called deutoplasm, in con- trast to the active and formative protoplasm. The pole of the ovum at which the formative protoplasm lies, and at which the spermatozoon enters, is often called the animal pole ; the other, towards which the heavier yolk tends to sink, is called the vegetative pole. In the floating ova of some fish, however, the yolk is uppermost, and the embryonic area lowest. In contrasting the chief modes of segmentation, it should be recognised that they are all connected by gradations. A. Complete Division — Holoblastic Segmentation (i) Eggs with little and diffuse yolk material di\ide completely into approximately equal cells, [or, Ova which are alecithal {i.e. without yolk) undergo approxi- mately equal holoblastic segmentation]. This is illustrated in most Sponges, most Coelentera (Fig. 39 (i)), some " Worms," most Echinoderms, some Molluscs, all Timicates, Amphioxiis, and most Mammals. (2) Eggs with considerable yolk material accumulated towards one pole, divide completely, but into unequal cells, [or, Ova with a considerable amount of deutoplasm lying towards one pole (telolecithal), undergo unequal holoblastic segmenta- tion]. This is illustrated in some Sponges, some Coelentera {e.g. Ctenophora), some " Worms," many Molluscs, the lamp- rey. Ganoid Fishes, Dipnoi, Amphibians (Fig. 39 (2)). CLEAVAGE OF THE OVUM 77 V • ♦ • «• * •* • * * • -V Fig. 39. — Modes of Segmentation. 1. Ovum, with little yolk, segments totally and equally into a blastosphere, e.g. Hydra, sponge, sea-urchin. 2. Ovum, with a considerable amount of yolk (y.) at lower pole, segments totally but unequally, e.g. 'frog ; {y.s.) larger yolk- laden cells. 3. Ovum, with much yolk (y.) at lower pole, segments partially and discoidally, forming blastoderm [bl.], e.g. bird, most fishes. Ovum, with central yolk (y.), segments partially and peripher- ally, e.^. most Arthropods. 78 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS B. Partial Division — Meroblastic Segmentation (3) Eggs with a large quantity of yolk on which the formative protoplasm lies as a small disc at one pole, divide partially, and in discoidal fashion, [or. Ova which are telolecithal. and have a large quantity of deutoplasm. undergo meroblastic and discoidal segmentation]. This is illustrated in all Cuttle-fishes, all Elasmobranch and Teleostean Fishes, all Reptiles and Birds (Fig. 39 (3)), and also in the Monotremes or lowest Mammals. (4) Eggs with a considerable quantity of yolk accumulated in a central core and surrounded by the formative protoplasm, divide partially, and superficially or peripherally, [or, Ova which are centrolecithal undergo meroblastic and peri- pheral segmentation.] This is illustrated by most Arthropods (Fig. 39 (4)), and by them alone. Cleavage pattern. — After fertilisation, and before the division of the egg into the two first " blastomeres," there may be a visible rearrangement of the materials in the cytoplasm. The subsequent cleavages very often follow so regular a pattern that it may be possible to point to a particular region of the cytoplasm of the ovum and predict the part which it is to play in the formation of the embryo, if development follows its normal course. But we must be chary of supposing that any such region is specialised from its surroundings except by position, for it is often possible to obtain complete embryos from fragments of eggs, or from isolated blastomeres from the two-cell or four-cell stage. Eggs which have this power of readjust- ing their organisation are called " regulative " in contrast to the " mosaic " eggs in which there is more evidence of the presence of a fixed structural pattern. Even here, however, it is too much to speak of " organ-forming substances " in the egg ; for it is usually found that the visible pattern of the cytoplasm may be completely changed by whirling the eggs in a centrifuge, without marked abnormalities in the subsequent development. The plane of the first cleavage is typically a meridian running through the two poles of the egg, and its exact situation is determined by the path of the spermatozoon nucleus and centrosome in the cytoplasm. For subse- quent cleavages, the simplest type is seen in the sea-urchin EXPERIMENTAL EMBRYOLOGY 79 and the frog, where the first three cleavage-planes are at right angles to each other, the first two being meridians and the third equatorial. In other cases the blastomeres may be unequal in size, so that a spiral type of cleavage results, and many intermediate forms are known. In insects and crustaceans there may be many nuclear divisions without division of the cytoplasm, though ultimately the nuclei separate and each becomes the centre of a cell. The two first blastomeres frequently correspond to the right and left halves of the future embryo, but exceptions to this are very common, and there may be much variation in this respect even within a single species. Conse- quently the problem of the origin of the bilateral sym- metry of the embryo is not the same as the problem of the determination of the first cleavage plane. The eggs of squids and of many insects have a bilateral shape and structure before fertilisation, and the axes of symmetry of the egg are the same as those of the embryo : cases have been described in which insect eggs are laid in lines, all pointing in the same direction, and the development can be followed through the transparent shell until there is an Indian file of unhatched larvae. Such eggs are enveloped in stiff membranes, and it is therefore likely that their symmetry is not inherent in their own structure but is imposed upon them by the maternal cells which produce the shell. In most other eggs, which are radially symmetrical before fertilisation, it seems that bilaterality is determined by the point at which the spermatozoon enters ; this holds, for instance, for the sea-urchin and frog. Experimental embryology. — Experiments on the power of isolated blastomeres to produce complete dwarf embryos have been made in almost all classes of animals. The sea-urchin's egg is an example of the most " regulative '* type : a blastomere representing only^ 73V of the egg has been known to develop normally for some time, and it seems possible that it is smallness of size rather than any regional differentiation that prevents the perfect develop- ment of these and smaller blastomeres. In most Coelenter- ates the blastomeres are " totipotent/' i.e. capable of producing a whole embryo, up to the four-cell stage, but 8o THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS in the Ctenophores certain structures are missing from the larvae which develop from isolated blastomeres. The eggs of Nematodes and Ascidians are of the mosaic type, and isolated blastomeres will not develop properly ; one member of the latter group, Styela, may be regarded as an extreme case ; the egg contains a yellow substance which is necessary for the formation of muscle-fibres, and if this substance is redistributed by centrifuging the development is upset. The case of the Amphibian egg, which has been much studied, is interesting. Diametrically opposite the point where the spermatozoon enters there forms a '* grey crescent " which eventually gives rise to the dorsal lip of the blastopore (see Fig. 39). This grey crescent is usually divided by the first cleavage plane, and in this case the blastomeres, if isolated, can give rise to whole embryos ; but if the cleavage-plane falls elsewhere, the blastomere which contains none of the material of the grey crescent will not develop. It is possible also to prevent or delay the development of one blastomere. at the two-cell stage, by injuring it with a hot needle ; the uninjured blastomere then behaves as if the sister-cell in contact with it were segmenting normally, and develops *' mosaically " into a half-embryo. Again, it may be possible to effect a partial separation of the two first blastomeres, for example by gentle shaking in the case of the egg of Amphioxus (Wilson) ; this may give rise to " Siamese- twin " or two-headed " Janus " embryos. The normal distribution of the nuclei in segmentation, e.g. in the frog or sea-urchin, may be completely upset by gentle pressure, yet normal embryos are produced ; this proves that during cleavage the nuclei of the blastomeres are identical, as far as their efl^ect on differentiation is concerned. Variations in the chemical composition of the medium may greatly affect the development of eggs. For example, the fish Fundulus in a solution containing magnesium chloride develops a single median " cyclopic " eye instead of the usual pair of eyes. If calcium, is absent from the medium, the blastomeres of t\\t sea-urchin's egg will not stick together, but become separated. In solutions GASTRULA 8 1 containing lithium sea-urchin eggs fail to gastrulate properly ; no invagination takes place, and abnormal hour- glass-shaped larvae are produced. Blastosphere and morula. — The result of the division is usually a ball of cells. But when the yolk is very abundant a disc of cells — a discoidal blastoderm — is formed at one pole of the mass of nutritive material, which it gradually surrounds. As the cells divide and redivide, they often leave a large central cavity — the segmentation cavity — and a hollow ball of cells — a blastosphere or blastula — results. But if the so-called " segmentation cavity " be very small or absent, a solid ball of cells or morula, like the fruit of bramble or mulberry, results* Gastrula. — The next great step in development is the establishment of the two primary germinal layers, the outer ectoderm and the inner endoderm, or the epiblast and the hypoblast. One hemisphere of the hollow ball of cells may be appar- ently dimpled into the other, as we might dimple an india- rubber ball which had a hole in it. Thus out of a hollow ball of cells, a two-layered sac is formed — a gastrula formed by invagination or embole (Fig. 40). The mouth of the gastrula is called the blastopore, its cavity the archenteron. But where the ball of cells is practically a solid morula, the apparent in-dimpling cannot occur in the fashion de- scribed above. Yet in these cases the two-layered gastrula is still formed. The smaller, less yolk-laden cells, towards the animal pole, gradually grow round the larger yolk-con- taining cells, and a gastrula is formed by overgrowth or epibole. In various ways the ectoderm and the endoderm are established, either by some form of gastrulation, or by some other process, such as that called de lamination. Mesoderm. — We are not yet able to make general state- ments of much value in regard to the origin of the middle germinal layer — the mesoderm or mesoblast. In Sponges and Coelentera it is not a distinct layer except in Cteno- phora, being usually represented by a gelatinous material {mesogloea), which appears between ectoderm and endoderm, and into which cells wander from these two layers. In the 6 82 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Other Metazoa, the middle layer may arise from a few Fk;. 40. — Life-history of a coral. Monoxenia darwinii. — From Haeckcl. A, B, Ovum. C, Division into two. D, Four-cell stage. E, Blas- tula. F, Free-swimming blastula with cilia. G, Section of blastula. H, Beginning of invagination. I, Section of com- pleted gastrula, showing ectoderm, endoderm, and archenteron. K, Free-swimming ciliated gastrula. primary mesoblasts or cells which appear at an early stage ORIGIN OF ORGANS 83 between the ectoderm and endoderm {e.s;. in the earth- worm's development) ; or from numerous " mesenchyme " immis^rant cells, which are separated from the walls of the blastula or gastrula {e.g. in the development of Echino- derms) ; or as ccelom pouches — outgrowths from the en- dodermic lining of the gastrula cavity {e.g. in Sagitia, Balanoglossus, Amphioxus) ; or by combinations of these and other modes of origin. The mesoderm lies or comes to lie between ectoderm and endoderm, and it lines the body cavity, one layer of mesoderm (parietal or somatic) clinging to the ectodermic external wall, theother(visceral or splanch- nic) cleaving to the endodermic gut and its outgrowths. Origin of organs. — From the outer ectoderm and inner endoderm, those organs arise which are consonant with the position of these two layers, thus nervous system from the ectoderm, digestive gut from the endoderm. The middle layer, w^hich begins to be developed in " Worms," assumes some of the functions, e.g. contractility, which in Sponges and Coelentera are possessed by ectoderm and endoderm, the only two layers distinctly represented in these classes. In a backboned animal the embryological origin of the organs is as follows :- - {a) From the ectoderm or epiblast arise the epidermis and epidermic outgrowths, the nervous system, the most essential parts of the sense organs, infoldings at either end of the gut (fore-gut or stomodasum and a trace of hind-gut or proctodasum). {b) From the endoderm or hypoblast arise the mid-gut (mesenteron) and the foundations of its out- growths {e.g. the lungs, liver, allantois, etc., of higher Vertebrates), also the axial rod or noto- chord. {c) From the mesoderm or mesoblast arise all other struc- tures, e.g. dermis, muscles, connective tissue, bony skeleton, the lining of the body cavity, and the vascular system. This layer aids in the formation of organs originated by the other two. With it the reproductive organs are associated. Con- nective tissues, vascular system, and unstriped muscles are formed by mesenchyme cells which are budded off from the true mesoderm. 84 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS Our knowledge of the origin of organs has been greatly- added to by the researches of Spemann, Mangold, and others on the development of Amphibians. Before gastrulation it is possible to exchange for a fragment of ordinary epidermis a fragment of that region of the epi- dermis which would normally be folded in to form the nerve-cord ; these fragments are found to be still " plastic," and the " presumptive " nerve-cord becomes ordinary skin, and the presumptive skin normal nerve- cord ; such transplants can even be made from one species to another. After gastrulation, however, although no visible differentiation has taken place, the plasticity has been lost and the exchange can no longer be made success- fully ; the fate of these regions of epidermis has been settled and sealed, presumably by some chemical alteration of the tissues. The " axial structures " on the back of the embryo, i.e. the nerve-cord, notochord, and somites, arise in the meridian of the dorsal lip of the blastopore as it travels backwards towards the vegetative pole. If, before this time, the upper part of the blastula is " scalped " off and replaced at right angles to its original position, the axial structures maintain their relation to the meridian of the dorsal lip, although the cells from which they are formed have been rotated. The position of the axial structures is determined by the dorsal lip of the blastopore, which, as we have seen, arises from the grey crescent of the egg. Still more remarkable, it is found that if the dorsal lip from one embryo is grafted into the flank of another, it will there induce the formation of an extra and imwanted set of axial organs, which would otherwise never have arisen ; these do not arise from the cells grafted-in, but from the cells of the host embryo under the influence of the implant. It is not even necessary that the implant should come from an embryo of the same species or genus as the host. The dorsal lip of the blastopore, which provokes the formation of a set of axial organs in normal or abnormal situations, is called the " organiser." The evidence suggests that from it chemical substances diffuse out into the surrounding tissues. The organiser cannot make its TRANSPLANTING EXPERIMENTS 85 influence felt across a cut where there is no contact ; and it is possible to graft indifferent tissue into contact with an oFganiser and infect it with organising properties. Similar transplanting operations have given much in- formation about the differentiation of various organs. For instance, the rudiment of the eye may be transplanted into such unlikely situations as the wall of the abdomen, and will there differentiate by itself into a typical optic cup. On the other hand, the development of the lens of the eye is determined by the presence of an optic cup behind the ectoderm from which the lens arises. Thus if the rudiment of the cup is transplanted, no lens arises in the " proper " place on the head whence the primordium has been removed, but a lens does form from the ectoderm covering the optic cup in its new situation in the abdomen. There may be curious differences in these respects between closely related species. Thus in one frog, Rana esculenta, ordinary epithelium will not form a lens ; but the optic cup of this species can provoke lens-formation from the ordinary epithelium of another species, such as R. fusca, in which lens-formation is dependent on the optic cup as described above. At a later stage in differentiation the function of the organs begins to play a part in differentiation. Thus the length of the intestine in tadpoles may be influenced by the diet. Again, in early stages the veins and arteries are alike, and it is possible to transplant a section of vein into the course of an artery, where eventually it becomes thick- walled and elastic under the influence of the higher blood- pressure ; whereas veins, where the blood-pressure is lower, are ordinarily thin-walled and flaccid. Generalisations. — (i) The ovum theory or cell theory. — All many-celled animals, produced by sexual reproduction, begin at the beginning again. " The Metazoa begin where the Protozoa leave off " — as single cells. Fertilisation does not make the egg-cell double ; there is only a more com- plex and more vital nucleus than before. All development takes place by the division of this fertilised egg-cell and its descendant cells. (2) The gastrcea theory. — As a two-layered gastrula stage occurs, though sometimes disguised by the presence of 86 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS much yolk, in the development of the majority of animals, Haeckel concluded that it represents the individual's re- capitulation of an ancestral stage. He suggested that the simplest many-celled animal was like a gastrula, and this hypothetical ancestor of all Metazoa he called a gastrcea. The gastrula is, on this view, the individual animal's recapitulation of the ancestral gastraea. Rival suggestions have been made : perhaps the original Metazoa were balls of cells like Volvox (Fig. 54), with a central cavity in which reproductive cells lay ; perhaps they were like the platmla larvae of some Coelentera — two-layered, externally ciliated, oval forms without a mouth. Fig. 41. — Embryos — (i) of bird ; (2) of man. — After His. The latter about twenty-seven days old. y.s., Yolk-sac ; pi., placenta. (3) The idea of recapitulation. — It is a matter of experi- ence that we recapitulate in some measure the history of our ancestors. Embryologists have made this fact very vivid, by showing that the individual animal develops along a path the stations of which correspond to some extent with the steps of ancestral history. (i) The simplest animals are single cells (Protozoa). (2) The next simplest are balls of cells {e.g. Volvox). (3) The next simplest are two- layered sacs of cells {e.g. (i) The first stage of development is a single cell (fertilised ovum). (2) The next is a ball of cells (blastula or morula). (3) The next is a two-layered sac of cells (gastrula). Hydra). Von Baer, one of the pioneer embryologists, acknow- ledged that, with several very young embryos of higher RECAPITULATION 87 Vertebrates before him, he could not tell one from the other. Progress in development, he said, was from a general to a special type. In its earliest stage every organism has a great number of characters in common with other organisms in their earliest stages ; at each successive stage the series of embryos which it resembles is narrowed. The rabbit begins like a Protozoon as a single cell ; after a while it may be compared to the young stage of a simple vertebrate ; then to the young stage of a higher vertebrate ; afterwards, to the young stage of almost any mammal ; afterwards, to the young stage of almost any rodent ; eventually it becomes un- mistakably a young rabbit. Herbert Spencer expressed the same idea, by saying that the progress of development is from homogeneous to heterogeneous, through steps in which the individual history is parallel to that of the race. But Haeckel has illustrated the idea more vividly, and summed it up more tersely, than any other naturalist. His " fundamental biogenetic law " reads : " Ontogeny, or the development of the individual, is a shortened recapitulation of phylogeny, or the evolution of the race." It is hardly necessary to say that the young mammal is never like a worm, or a fish, or a reptile. It is at most like the embryonic stages of these, and it may also be noticed that, as our knowledge is becoming more intimate, the individual peculiarities of different embryos are be- coming more evident. But this need not lead us to deny the general resemblance. Moreover, the individual life-historv is much shortened compared with that of the race. Not merely does the one take place in days, while the other has progressed through ages, but stages are often skipped, and short cuts are dis- covered. And again, many young animals, especially those " larvae " which are very unlike their parents, often exhibit characters which are secondary adaptations to modes of life of which their ancestors had probably no experience. In short, the individual's recapitulation of racial history is general, but not precise. It is seen rather in the stages in the development of organs (organogenesis) than in the development of the organism as a whole. 88 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS (4) Organic continuity between generations. — Heredity. — Everyone knows that like tends to beget like, that offspring resemble their parents and their ancestors. Not only are the general characteristics reproduced, but minute features, idiosyncrasies, and pathological conditions, inborn in the parents, may recur in the offspring. At an early stage in the development of the embryo the j^B*^ .^^^iv^^mi^^iXS:;^^^^) *..... ..lI^L ^ ^ uW^'m>H ^- ^ ^:^^.I^;^-;^..^^^^^^ Fig. 42. — Larvae of Common EeL (Not drawn to scale.) I. Smallest l-nown larva, 7 mm. in length. II. and III. Laterally compressed transparent Leptocephalus stages. I\'. Change, when about two years old, from knife-blade-like to cylindrical shape. \'. Young elver, about three inches long, about two and a half years old. future reproductive cells of the organism are often dis- tinguishable from those which are forming the body. These, the somatic cells, develop in manifold variety, and, as division of labour is established, they lose their likeness to the fertilised ovum of which they are the descendants. The future reproductive cells, on the other hand, are not implicated in the formation of the " body," but, remaining virtually unchanged, continue the protoplasmic tradition unaltered, and are thus able to start an offspring which SEGREGATION OF GERM-CELLS 89 will resemble the parent, because it is made of the same protoplasmic material, and develops under similar con- ditions. An early isolation of reproductive cells, directly con- tinuous and therefore presumably identical with the original ovum, has been observed in the development of some *' worm types" — {Sagitta, Thread -worms, Leeches, Polyzoa), and of some Arthropods {e.g. Moina among Crustaceans, Chironomus among Insects, Phalangidae among Arachnids), Micrometnis aggregatus among Teleo- stean fishes, and with less distinctness in some other animals. A cell which will give rise to the germ-cells can be recognised in the gastrula stage of Cyclops, and in the very first segmentation stages of the thread-worm Ascaris. In many cases, however, the reproductive cells are not recognisable until a relatively late stage in development, after differentiation has made considerable progress. Weismann got over this difficulty by supposing that the continuity is sustained by a specific nuclear substance — the germ-plasm — which remains unaltered in spite of the diflferentiation in the body. It is perhaps enough to say that, as all the cells are descendants of the fertilised ovum, the reproductive cells are those which retain intact the qualities of that fertilised ovum, and that this is the reason why they are able to develop into off'spring like the parent. Finally, it may be noticed in connection with heredity, that there is great doubt to what extent the " body " can definitely influence its own reproductive cells. Animals acquire individual bodily peculiarities in the course of their life, as the result of what they do or refrain from doing, or as dints from external forces. The " body " is thus changed, but there is much doubt whether the repro- ductive cells within the " body " are affected specifically hy such changes. Weismann denied the transmissibility of any characters except those inherent in the fertilised egg- cell, and therefore denied that the influences of function and environment are, or have been, of direct importance in the evolution of many-celled animals. Such influences affect the body, and produce what are technically called " modifications,'' but these modifications do not affect the 90 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS reproductive cells — at least not in a specific representative way. Therefore modifications are not likely to be trans- mitted, and there seems no good evidence to show that they are. Many of the most authoritative biologists are at present of this opinion. On the other hand, many still maintain that profound changes due to function or environ- ment may saturate through the organism, and affect the reproductive cells in such a way that the changes or modifications in question are in some measure transmitted to the next generation. The question remains under dis- cussion, but the probabilities are strongly against the transmissibility of acquired characters. It is important to try to distinguish different modes of hereditary resemblance. The characters of the two parents may be blended in the offspring, or those of one parent may find predominant expression {exclusive inheritance), or the characters of one parent may be expressed in one part of the offspring and those of the other parent in another {particulate inheritance). Another important inquiry is into the share that the various ancestors have on an average in forming any indi- vidual inheritance. The inheritance of an animal repro- duced in the ordinary way is always dual, partly maternal and partly paternal, but through the parents there come contributions from grandparents, etc. Galton's Law of Ancestral Inheritance states that " The two parents con- tribute between them, on the average, one half of the total heritage ; the four grandparents, one quarter ; the eight great-grandparents, one eighth, and so on." Mendelian inheritance. — Of the greatest practical and theoretic importance in the study of heredity are the laws discovered by Mendel in 1865, but almost ignored until 1900. In their original form these laws are empirical formulations of the average results of breeding experi- ments ; but since 1900 the hypothetical basis suggested by Mendel to explain these laws has become much more concrete and definite. If black Andalusian and white Andalusian fowls be bred together, the offspring (the first filial or " Fj " generation) bear a finely divided pattern of black-and- white markings which gives a blue effect. But if two of MENDELIAN INHERITANCE QI these blue Andalusian fowls be mated, their offspring (the " F2 " generation) are not all blue ; some are black and others are white. On the average, in this generation, there will be equal numbers of pure black and pure white, and twice as many of the mixed or blue form. Far more commonly, however, it is found that there is no blending of the contrasted parental characters, but that one prevails over the other. Thus when a black guinea-pig is mated with a white one, the offspring in the Fi ccf) Fn /^ Aa A A A a ^^ . > r ^— AA AA Aa aA aa AA Aa aA 0.0. aa Fig. 43. — Mendelian inheritance illustrated in wood snail [Helix nemoralis). P., The parents, bandless [A), dominant, and banded {a), recessive. F.I., First filial generation, all bandless (.-ta). F.ii., Second filial generation, 25 per cent, pure bandless {A A), yielding bandless offspring in the next generation (F.iii.). 25 per cent, pure banded {aa), yielding banded offspring in the next generation (F.ni.). 50 per cent, bandless {Aa), with the banded character recessive as in F.i. These, if inbred, yield in the F.iii. generation the same ratio : lAA + zAa + iaa. F^ generation are not intermediate in colour, but perfectly black ; blackness is said to be dominant over whiteness or albinism, which is a recessive character. In the Fg generation the majority are again pure black and the minority pure white, the ratio between blacks and whites being three to one. Now if these white guinea-pigs of the Fg generation be inbred, their offspring will always be pure white ; in the same way either the black or white Anda- lusian fowls may be inbred without any other shade of 92 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS colour appearing. But if the black guinea-pigs of the Fg generation are inbred, a fraction of their offspring will be white. Further examination will show that the F2 genera- tion of guinea-pigs, with their three-to-one ratio of black to white, is more comparable than appears at first sight to the F2 generation of the Andalusian fowls, with their one-two-one ratio of black, blue, and white. For if the blue Andalusian fowls happened to be more like the black ones than they are, so like that they could not be distin- guished, then it would not be possible, except by chance, to inbreed the pure blacks of the Fg generation and obtain only pure black offspring ; one would be more likely to have chosen one or two of the indistinguishable mixed forms, and a fraction of the offspring would be white. Thus it appears that of the black guinea-pigs of the Fg generation one-third are " pure " black, and if by chance these can be inbred, their offspring will all be black; but two-thirds of the Fg black guinea-pigs are in reality mixed forms, and if two of them can be bred together, their offspring will be blacks and whites again in the ratio three-to-one. But the pure blacks and the mixed blacks are indistinguishable in appearance, and only by lengthy breeding experiments can the one be told from the other. The matter may be made clearer by a diagram, in which " D " represents the dominant character and " R " the recessive, which is completely masked in the presence of the dominant. P generation D x R Fi generation /D(R) \ V \ F2 generation D D(R) D(R) R / ■ ^ \ D D D(R) D(R) R R To explain these results it is suggested that the mixed forms of the F^ generation receive a " factor " for blackness from the one parent, a factor for whiteness from the other, though the presence of the latter is masked. But the recessive factor remains present, and the germ-cells of the MENDELISM 93 Fj generation are of two different kinds in each sex ; thus the Fi females have two kinds of ova, equal numbers of each : one kind containing the factor for blackness only, the other that for whiteness only. Similarly the sper- matozoa of the males are of two kinds, containing either factor, and again equal numbers of each. In the inbreed- ing of the Fj generation these germ-cells may be supposed to come together at random, and the result will be the formation of equal numbers of four kinds of fertilised egg-cells : *' black " ovum and " black " spermatozoon, '' black " and " white," " white " and " black," *' white" and " white." The second and third of these are identical, and the animals developing from them will be identical in appearance with those developing from the first kind of fertilised ovum ; only by continued breeding will the presence of the recessive " white " factor become manifest. In short, if the constitution of the F^ generation is expressed as BW, the germ-cells are either J5 or Win the female, B or W in the male, and the following combinations result : BB, BW, WB, WW, all the forms in which the dominant character is present being apparently identical. When Mendel's work was rediscovered it was seen that precision could be given to the theory by the assumption, since then abundantly justified, that the hereditary " factors " were carried in some way by the chromosornes. For we know that in the cells of the F^ generation the chromo- somes are half of paternal, half of maternal origin ; and that in the development of the spermatozoa a reducing division takes place, in which the chromosomes are sorted out into two lots, and the paternal chromosome which bears the factor for colour is separated from the corre- sponding one of maternal origin, so that there will be two kinds of spermatozoa, differing in this particular ; and that a similar reducing division in the maturation of the ovum will have a similar effect, producing equal numbers of two kinds of ova, if it is a matter of chance whether it is the paternal or the maternal chromosome that is thrown out in the first polar body. The factor for a given character carried by a chromosome is now often called a '* gene " and regarded as a specific material substance, but speculations as to its nature or mode of 94 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS action are as yet hardly profitable. In the case discussed we know that the black pigment of the guinea-pig's coat (melanin) is formed by the action of an enzyme (tyrosinase) on a colourless precursor, and that in the albinos the enzyme is lacking ; we may perhaps regard the " gene " for blackness as a substance essential for the formation of the enzyme, and the character for whiteness as merely the absence of this substance ; then any animal which has received the enzyme-forming gene, from either parent or from both, will be black. It is demonstrable that sex is inherited on Mendelian principles, and the idea has important applications. The inheritance of sex is to be compared with the result of crossing a mixed or DR form with a pure recessive or RR form, a " back-cross." Then the germ-cells of one parent are of two kinds, of the other parent all alike, and the offspring will again be of two kinds, DR or RR, in equal numbers. It has been demonstrated for a large number of species that the cells of the female contain two recognisable " X " chromosomes, while those of the male contain only one X-chromosome and, corresponding to the other, a small Y-chromosome : so that the males may be regarded as mixed forms in this respect, and every mating as a '* back-cross " between XX and XY. In other cases the Y-chromosome may be altogether absent ; and in the Lepidoptera and probably in birds it is the females that are mixed, so that they have two kinds of eggs, male- producing and female-producing, while other animals have two kinds of spermatozoa. Mendel's second great discovery was that when parents differing in two characters were mated, the offspring inherited these characters independently. Thus, in guinea-pigs, " rough-coat " is dominant over " smooth- coat " : so that when a black smooth-coated guinea-pig is crossed with a white rough-coated guinea-pig, all the members of the F^ generation are black and rough-coated. In the F., generation the rough-coats outnumber the smooth- coats in the ratio three to one, just as the blacks, pure and apparent, outnumber the whites ; but the two characters are independent of each other. Thus of every four black F.J guinea-pigs, three are rough-coated ; and of DIHYBRIDISM 95 every four smooth-coated, only one is white, on the average. The Fo generation cannot be truly represented by a group of four animals any longer, but only by a group of sixteen : of these, nine show both dominant characters, blackness and rough-coat ; three are black and smooth-coated, three are white and rough-coated, and only one shows both recessive characters, whiteness and smooth-coat. On the chromo- some theory it is supposed that the two contrasting pairs of characters are borne by different chromosomes, and further that in the reducing divisions it is a matter of chance whether it is the paternal or the maternal coat-deter- mining chromosome that accompanies the paternal colour- determining chromosome when it separates from its partner. Thus it follows that in the Fj generation there are four kinds of ova and four kinds of spermatozoa, and consequently sixteen possible combinations, though many of these are identical in effect. This may be represented by a diagram, in which B represents black, W white, R rough-coat, and S smooth-coat ; small letters are used for the recessive characters where they are present, but concealed by the presence of the dominant. p BBSS X WWRR Fi B wR? F2 BBRR BBRs; BwRR BwRs BBRs BBSS BwRs BwSS BwRR BwRs WWRR WWRs • BwRs BwSS WWRs WWSS When the chromosome theory was applied to Mendelism, Boveri predicted that exceptions to Mendel's second law would be found, for in cases where the genes for two different sets of characters happened to lie on the same chromosome they w^ould not be separated in the reducing divisions, and the two characters would not be inherited independently, but together. This was confirmed by Bateson and Punnett, who, like Mendel, studied the inheritance of characters in the pea, and subsequently by many other workers on other subjects. The vinegar-fly, Drosophila melanogaster and related species, has been much used for such studies by Morgan and his colleagues, for it has the advantages of being prolific and easily reared, of being remarkably variable in very many characters, and 96 THE REPRODUCTION AND LIFE-HISTORY OF ANIMALS of having a simple chromosome complex (only four pairs). For instance, in Drosophila, the characters for black body- colour, as opposed to the grey wild type, and for vestigial or badly developed wings, both recessives, are linked together and are thus known to lie on the same chromosome; one never occurs without the other. Further analysis shows that all the variable characters of Drosophila can be arranged in four linkage groups, corresponding to the four chromosome pairs. There is a special interest in the cases, long known in man though not understood, where characters lie upon the " X " or sex-chromosome, and are said to be sex-linked. Thus if a man suffers from haemophilia, a congenital tendency to excessive bleeding, his spermatozoa will be of two kinds : half carry the X-chromosome and with it the gene for haemophilia (which is a recessive character), and may give rise to females who themselves do not show the condition but pass it on to half of their male offspring, and half the spermatozoa carry the Y-chromosome, on which there are few or no genes ; these may give rise to males, who will be free from the haemophilia altogether, so that it cannot appear in their offspring. The studies on Drosophila have demonstrated that exceptions to the law of linkage may occur. Before the reducing division in the maturation of the ova the chromo- some-pairs are tightly interlaced, and an exchange of material or " crossing-over " may take place between them. In this way a gene may pass from the chromosome on which it originally lay to the partner chromosome, and two linked characters may be separated. As the genes do not pass over singly but in groups, the frequency of separa- tion of linked characters is a measure of the distance between their genes on the chromosome, and in this way Morgan and his colleagues have been able to prepare chromosome maps for Drosophila^ showing the position on the chromosomes of the genes of over three hundred mutant characters and the relative distance between them. The chromosome maps of related species are closely similar. Conclusion. — Heredity may be defined as the relation of genetic continuity between successive generations, and HEREDITY 97 inheritance as all that the organism is or has to start with in virtue of this hereditary relation. Development is the expression or realisation of the heritable qualities which have their physical basis in the germ-cells, and it pre- supposes an appropriate environment of nutrition and " hberating stimuli " — " nurture " in the widest sense. What the organism becomes is the resultant of two com- ponents, inherited " nature " and external " nurture." CHAPTER V PAST HISTORY OF ANIMALS (Palaeontology) In the two preceding chapters we have noticed two of the great records of the history of animal Hfe — that preserved in observable structures, and the modified recapitulation discernible in individual development ; in this we turn to the third — the geological record. In the early days of the Evolution theory the modern science of Embryology was still in its infancy, and could furnish few arguments, and it was the opponents of the new theory rather than its sup- porters who appealed to Palaeontology. They asserted that the palaeontological facts refused to lend the support which the theory demanded. To their attacks the evolutionists usually replied by pointing out that the geological record was very incomplete. The numerous investigations which have since been carried on on all sides now show con- clusively that it was imperfection rather of knowledge than of the record which produced the negative results. We must, however, still acknowledge that, except in a few cases, there is but little certainty as to the precise pedi- gree of living animals, and seek for reasons to explain this. "Imperfection of the geological record." — If we re- member the rule of modern Geology, that the past is to be interpreted by the aid of the present, there can be no difficulty in realising that the chances against the preserva- tion of any given animal are very great. Many are destroyed by other living creatures, or obliterated by chemical agencies. Except in rare instances, only hard parts, such as bones, teeth, and shells, are likely to be preserved, and this at once greatly limits the evidential value of fossils. The primitive 98 FOSSILS 99 forms of life would almost certainly be without hard parts, and have left no trace behind them. A number of ex- tremely interesting forms, such as many worms and the Ascidians, are, for the same reason, almost unrepresented in the rocks. Finally, we cannot suppose that such an external structure as a shell can always be an exact index of the animal within. After fossilisation has taken place, the rock with its con- tents may be entirely destroyed by subsequent denudation, or so altered by metamorphic changes that all trace of organic life disappears. Of those fossils which have been preserved only a small percentage are available, for vast areas of fossiliferous rocks are covered over by later deposits, or now lie below the sea or in areas which have not yet been explored. With all these causes operating against the likelihood of preservation, and of finding those forms that may have been preserved, it is little wonder if the geological record is incomplete ; but such as it is, it is in general agreement with what the other evidence, theoretical and actual, leads us to expect as to the relative age of the great types of animal life. Further, those specially favourable cases which have been completely worked out have yielded results which strongly support the general theory. Probabilities of ** fossils." — But it will be useful to note the probabilities of a good representation of extinct forms in the various classes of animals. Thus among the Protozoa the Infusoria have no very hard parts, and have therefore almost no chance of preservation, and the same may be said of forms like Amoeba ; while the Foramin- ifera and the Radiolaria, having hard structures of lime or silica, have been well preserved. The flinty Sponges are well represented by their spicules and skeletons. Of the Coelentera, except an extinct order known as Graptolites, only the various forms of coral had any parts readily capable of preservation, and remains of these are very abundant in the rocks of many ancient seas. But, strange as it may seem, some beautiful vestiges of jelly-fish have been discovered. Of the great series of " worms," only the tube-makers have left actual remains ; the others are known only by their tracks, while of any that may have lived on the land there is no evidence. The Echinoderms, because of their hard parts, are well represented in all their orders, except the Holothurians, where the calcareous structures characteristic of the class are at a minimum. The Crustacea, being mostly aquatic, and in virtue of their hard shells, are fossilised in great numbers. lOO PAST HISTORY OF ANIMALS The Arachnida and the Insects, owing to their air-breathing habit, are chiefly represented by chance individuals that have been drowned, or enclosed within tree-stumps and amber. The Molluscs and Brachiopods are perhaps better preserved than any other animals, since nearly all of them are possessed of a shell specially suitable for preservation. Among the Vertebrates some of the lowest are without scales, teeth, or bony skeleton ; such forms have therefore left almost no traces. Fishes, which are usually furnished with a firm outer covering, or with a bony internal skeleton, or with both, are well represented. The primitive Amphibians were furnished with an exoskeleton of bony plates, and are fairly numerous as fossils. The bones and teeth of the others have been fossilised, though more rarely. Of some the only record is their footprints. The traces of Reptilia depend upon the habits of the various orders, those living in water being oftenest preserved, but the strange flying Reptiles have also left many skeletons behind them. Of the Birds, the wingless ones are best represented, and then those that lived near seas, estuaries, or lakes. The history of Mammals is very imperfect, for most of them were terrestrial. But the discoveries of Marsh, Cope, and others show how much may be found by careful search. The aquatic Mammals are fairly well preserved. " Palaeontological series." — In spite of the imperfec- tion of the " geological record," in spite of the conditions unfavourable to the preservation of many kinds of animals, it is sometimes possible to trace a whole series of extinct forms through progressive changes. Thus a series of fossilised fresh -water snails (Planorbts) has been worked out ; the extremes are very different, but the intermediate forms link them indissolubly by a marvellously gradual series of transitions. The same fact is well illustrated by another series of fresh- water snails {Paludina, Fig. 44), and not less strikingly among those extinct Cuttle-fishes which are known as Ammonites, and have perfectly preserved shells. Similarly, though less perfectly, the modern crocodiles are linked by many intermediate forms to their extinct ancestors, for it is impossible not to call them by that name. In short, as knowledge increases, the evidence from Palaeontology becomes more and more complete. In a general way it is true that the simpler animals pre- cede the more complex in history as they do in structural rank, but the fact that all the great Invertebrate groups are represented in the oldest distinctly stratified and fossili- ferous rocks — the Cambrian system — shows that this corre- EXTINCTION OF TYPES lOI spondence is only roughly true. To account for this, we must remember that almost the whole mass of the oldest rocks, known as Archaean or Pre-Cambrian, has been so profoundly altered, that, as a rule, only masses of marble and carbonaceous material are left to indicate that forms of life existed when these rocks were laid down. Careful searching in Pre-Cambrian beds has revealed the presence of several Molluscs, a Eurypterid, and a fragment of Trilobite. There are also ** annehd tracks " indicative of life. Extinction of types. — Some animals, such as some of the lamp-shells or Brachiopods, have persisted from almost Fig. 44. — Gradual transitions between Paludma neumayri (a), the oldest form, and Paludina hcernesi (;'). — From Neumayr. the oldest ages till now, and most fossilised animals have modern representatives which we believe to be their actual descendants. That a species should disappear need not surprise us, if we believe in the " transformation " of one species into another. The disappearance is more apparent than real : the species lives on in its modified descendants, " different species " though they be. But, on the other hand, there are not a few fossil animals which have become wholly extinct, having apparently left no direct descendants. Such are the Graptolites, the ancient Trilobites, their allies the Eurypterids, two classes of Echinoderms (Cystoids and Blastoids), many giant Reptiles, and some Mammals, 102 PAST HISTORY OF ANIMALS It is almost certain that there has been no sudden extinction of any animal type. There is no evidence of universal cataclysm, though local floods, earthquakes, and volcanic eruptions occurred in the past, as they do still, with disastrous results to fauna and flora. In many cases the waning away of an order, or even of a class of animals, may be associated with the appearance of some formidable new competitors ; thus cuttle-fish would tend to exter- minate Trilobites, just as man is rapidly and often inex- cusably annihilating many kinds of beasts and birds. Apart from the struggle with competitors, it is con- ceivable that some stereotyped animals were unable to accommodate themselves to changes in their surroundings, and also that some fell victims to their own constitutions, becoming too large, too sluggish, too calcareous — in short, too extreme. Appearance of animals in time. — Such tables as those given here are apt to be misleading, in that they convey the impression that the great types of structure have appeared suddenly. It must be noted that any apparent abruptness is merely due to incompleteness of knowledge or inaccuracy of expression. The table is a mere list of a few important historical events, but one must fully realise that they are not isolated facts, that the present lay hidden in the past and has gradually grown out of it. Of the relative length of the periods represented here we know almost nothing, and we are also ignorant of the earliest ages in which life began. But the general result is clear. We find that in the Cambrian rocks, before Fishes appeared, the great Invertebrate classes were represented, though as yet but feebly. As we pass upwards they increase in number and in differentiation. Again, Fishes precede Amphibians, Amphibians are historically older than Reptiles, and many types of Reptiles are much older than Birds. In short, in the course of the ages life has been slowly creeping upwards. [Tables GEOLOGICAL SUCCESSION 103 Quaternary or Post-Tertiary. Pliocene. > Ganoids and Elasmo- branchs. •1 Ordovician. Cambrian. Representa- tives of aD the chie) classes oi I n v e r t e- brates. Pre-Cambrian or Archaean. 104 PAST HISTORY OF ANIMALS Coelentera. Echinoderma. Arthropoda. Cephalopoda. ' S . ^ > , ■ > ' — ^ Quaternary or Post- Tertiary. - - — - Pliocene. in B u a d o u c n « •S 2 Miocene. H 'O • • ; Eocene. *o a; ■4-t < (A 'o 1 u5 o o u Cretaceous. O . - <0 ^^ -^ *^ O •^ p Jurassic. ^5 "S . . . rt u _o a. o < 'c S to Triassic. Permian. Carboniferous. in 1-1 C "c '/i ... u u o Devonian or -1 Old Red Sandstone. a. o ^ ' Silurian. •*4 c/5 u 'c ■4-* 'u r-' K ft, Ordovician. Cambrian. Pre-Cambrian or Archaean. CHAPTER VI THE DOCTRINE OF DESCENT When we ask, as we are bound to ask, how the hving plants and animals that we know have come to be what they are — very numerous, very diverse, very beautiful, marvellous in their adaptations, harmonious in their parts and qualities, and approximately stable from generation to generation — we may possibly receive three answers. According to one, the plants and animals that we know have always been as they are ; but this is at once contradicted by the record in the rocks, which contain the remains of successive sets of plants and animals very different from those which now live upon the earth. According to another, each successive fauna and flora was destroyed by mundane cataclysms, to be replaced in due season by new creations, by new forms of life which arose after a fashion of which the human mind can form no conception. Of such cataclysms there is no evidence, and if it be enough to postulate one creation, we need not assume a dozen. The third answer is, that the present is the child of the past in all things : that the plants and animals now existing arose by a natural evolution from simpler pre-existing forms of life, these from still simpler, and so on back to a simplicity of life such as that now represented by the very lowest organisms. This third theory is really an old one ; it is merely man's application of his idea of human history to the world around him. It was maintained with much concreteness and power by Buffon (1749), by Erasmus Darwin (1794), and by Lamarck (1801). Yet in spite of the labours of these thoughtful naturalists and of many others, the general idea of the natural descent of organisms from simpler ancestors was not received with favour until Darwin, in his Origin 105 I06 THE DOCTRINE OF DESCENT of Species (1859), made it current intellectual coin. By his work, and by that of Spencer, Wallace, Haeckel, Huxley, and many others, the doctrine of descent, the general fact of evolution, has been established, and is now all but universally recognised. The chief arguments which Darwin and others have elaborated in support of the doctrine of descent, according to which organisms have been naturally evolved from simpler forms of life, may be ranked under three heads — {a) structural, (b) physiological, {c) historical. Evidences of evolution. — {a) Structural. — Some say that there are over a million living animals of different species. In any case, there are many myriads. These species are linked together by varieties which make strict severance often impossible ; they can be rationally arranged in genera, orders, families, and classes, between which there are not a few remarkable connecting links ; there is a gradual increase of complexity from the Protozoa upwards along various hues of organisation ; it is possible to rank them all on a hypothetical genealogical tree (Fig. 18). A little practical experience makes one feel that the facts of classification favour the idea of common descent. Throughout vast series of animals we find in different guise essentially the same parts twisted into most diverse forms for different uses, but yet referable to the same funda- mental type. It is difficult to understand this " adherence to type," this " homology " of organs, except on the theory of natural relationship. There are many rudimentary organs in animals, especially in the higher animals, which remain very slightly developed, and which often disappear without having served any apparent purpose. Such are the " gill-slits " or " visceral clefts " in Reptiles, Birds, and Mammals, the teeth of young whalebone whales, the pineal body (a rudimentary eye) in Vertebrates. Only on the theory that they are vestiges of structures which were of use in ancestors are these rudi- ments intelligible. They are relics of past history, com- parable, as Darwin said, to the unpronounced letters in many words. (b) Physiological. — Observation shows that animals are to some extent plastic. In natural conditions they usually EVIDENCES OF EVOLUTION IO7 exhibit some measure of changefulness from generation to generation. This is especially the case if one section of a species be in any way isolated from the rest, or if the animals be subjected in the course of their wanderings to novel conditions of life. The evidence from domesticated animals is very convinc- ing. By careful interbreeding of varieties which pleased his fancy or suited his purpose, man has produced numerous breeds of horses, cattle, sheep, and dogs, which are often distinguished from one another by structural differences more profound than those which separate two natural species. In great measure, however, domestic breeds are fertile with one another, while different species rarely are. The numerous and very diverse breeds of domestic pigeons, which are all derived from the rock-dove {Columba livia), vividly illustrate the plasticity or variability of organisms. It sometimes happens that offspring resemble not so much the parent as some other form believed or known to be ancestral. Thus a pigeon like the known ancestor Columba livia may be hatched in the dovecot, and a few instances are known of similar reversions to a presumed ancestor. {c) Historical. — Among the extinct animals disentombed from the rocks, many form series by which those now existing can be linked back to simpler ancestors. Thus the ancient history of horses, crocodiles, and cuttle-fish is known with a degree of completeness which makes it almost certain that the simpler extinct forms were in reality the ancestors of those which now live. Moreover, many con- necting links have been discovered in the rocks, and the higher animals appear gradually in successive periods of the earth's history The facts of geographical distribution, and the history of the diffusion of animals from centres where the presumed ancestral forms are or were most at home, favour the doctrine of descent. The individual life-history of an animal — often strangely circuitous or indirect — is interpretable as a modified re- capitulation of the probable history of the race. Such, in merest outline, is the nature of the evidence which leads us to conclude that the various forms of life I08 THE DOCTRINE OF DESCENT have descended or have been evolved from simpler an- cestors, and these from still simpler, and so on, back to the mist of life's beginnings. None of the evidence is logically demonstrative ; we accept the evolution idea because it is a plausible interpretation which is applicable to many orders of facts, and is contradicted by none. In accepting the evolutionist interpretation naturalists are unanimous ; but in regard to the manner in which the transformation of species or the general ascent of life has been brought about, there is much difference of opinion. The fact of evolution is admitted ; debate goes on with regard to the factors (see Chapter XXVIII). CHAPTER VII PHYLUM PROTOZOA— THE SIMPLEST ANLMALS Chief Divisions Rhizqpods : Classes — Lobosa, Heliozoa, Foraminifera, Radio- LARIA, etc. Infusorians : Classes — Flagellata, Ciliata, Acinetaria, etc. Sporozoa : Several Classes. The Protozoa are the simplest animals, and they are of pecuhar interest on this account. They throw light upon the beginnings of organic structure and vital activity, and they give us hints as to the nature of the first forms of life, of which we can know nothing directly. Almost all the Protozoa are single cells, iinit masses of living matter ; and in virtue of their simpUcity, they are in some measure exempt from natural death, which is " the price paid for a body." In their variety they exhibit, as it were, a natural analysis of the higher animals, w^hich are built up of many diverse cells. General Characters The Protozoa^ the simplest and -most primitive animals, are usually very small single cells. Most of them feed on small plants or on other Protozoa, or on debris, and not a few are parasitic. Most of them live in water, hut many can endure dryness for some time. In one series (Rhizopods) the living matter is without any rind, and flows out in more or less changeful threads and lobes, by the movements of which the animals engulf their food and glide along. The others 109 no PHYLUM PROTOZOA — THE SIMPLEST ANIMALS have a definite rind, which in a large number (Infusorians) bears motile cilia or flagella, but in the others (Sporozoa) is usually without locomotor structures. But these three phases — amoeboid, ciliate or flagellate, and encysted— may occur in the life-history of one form ; and the three main lines of evolution — Rhizopods, Infusorians, and Sporozoa^are marked by the predominant occurrence of the amoeboid, ciliate or flagellate, or encysted phase of celt life. Many have a skeletal framework— of lime, flint, or other material— while within the cell there is a special kernel or nucleus, or there may be several. There are also other less constant structures. A Protozoon multiplies by dividing into two daughter units, or into a large number ; and two individuals often unite, temporarily or permanently, in conjugation, which is analogous to the union of ovum and spermatozoon in higher animals. A few types, instead of remaining single cells, form by division or budding loose colonies, taking a step, as it were, tozvards the Metazoa, but never forming differentiated tissues. First Type of Protozoa — Amceba Amoeba, a type of Rhizopods, especially of those in which the outflowing processes of living matter (pseudopodia) are blunt and finger-like (Lobosa). Description. — Amoeba proteus and some other species are found in the mud of ponds ; A. terricola occurs in damp earth. Some are just large enough to be seen with the unaided eye. The diameter is often about one-hundredth of an inch. Each is a unified corpuscle of living matter, and glides over the surface of stone and plant by protruding and retracting the pseudopodia. As they move the shape constantly changes, whence the old (1755) name of " Pro- teus animalcule." Round the margin, which may show an apparent radial striation, the cell substance is firmer and clearer than it is in the interior, where it is more fluid, but contains very abundant granules, some of which are of a protein, and others of a fatty nature. In the centre of the cell lies the usually single nucleus. The food consists of minute Algae, such as diatoms, or of vegetable debris. There is reason also to suspect cannibalism. The food is surrounded by the finger-like processes, and engulfed STRUCTURE OF AMCEBA III along with drops of water, which form food vacuoles in the cell substance. Into these vacuoles digestive ferments flow ; D Fig. 45. — Amceba proteus. — After Lucy A. Carter. EC, Clear ectoplasm, sometimes showing fine radial striation. EN., More granular endoplasm. The granules are partly nutritive substances in reserve, partly waste products and undigested debris. F.V., Food vacuoles, droplets of water surrounding food particles. A solid particle is shown in process of digestive disintegration. C.V., A contractile vacuole with excretory function. D., A diatom that has been engulfed as food. PS., A pseudopodium. N., The nucleus showing chromatin bodies (represented as dark granules). the contents of the vacuoles are first acid and then alkaline, which recalls the change of reaction in the alimentary canal in mammals ; but it is doubtful if there is true 112 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS digestion during the acid phase in Amoebae. The fer- ments secreted attack proteins, carbohydrates including cellulose, but possibly not fats. After the digestible parts of the food have been absorbed, the undigested residue is got rid of at any point of the protoplasm. One or more contractile vacuoles are visible in the cell substance. They have an excretory function, and serve to get rid of the finer waste products, and also of the w^ater which must be continually drawn into the cell, whose contents have a higher osmotic pressure than the media in which the Amoebae usually live. Life-history. — In favourable nutritive conditions the Amoeba grows. At the limit of growth it reproduces by Fig. 46. — Life-history of ^mQ?&a. I. Amoeba with pseudopodia ; n., nucleus ; c.v., contractile vacuole. 2. Division in two. 3. Encystation. 4. Escape of Amoeba from its cyst. dividing into two. In disadvantageous conditions, such as drought, it may become globular, and, secreting a cell wall or cyst, lie dormant for a time. The cyst wall is said to be chitinoid. With the return of favourable conditions the Amoeba revives, and, bursting from the cyst with renewed energy, recommences the cell-cycle. The conjugation of two Amoebae has been observed, and spore-formation occasionally occurs. Second Type of Protozoa — Actinophrys The Sun-animalcule, Actinophrys sol, is a type of the Heliozoa. Description. — Like most other Heliozoa, Actinophrys lives in fresh water, floating about or rolling over the SUN-ANIMALCULE 113 bottom. It is spherical and minute, measuring at most 0-05 mm. in diameter. Long stiff pseudopodia radiate out from the body. A clear axial filament runs up each pseudopodium, and the small organisms on which Actino- phrys feeds are paralysed when they come in contact with the pseudopodia. The body consists of ectoplasm and endoplasm. The ectoplasm is a thick external layer closely packed with large vacuoles, which are non-contractile and contain a clear fluid. But food vacuoles are formed as in other Fig. 47. — Actinophrys sol (Sun-animalcule). — After Grenacher. »., Nucleus ; f.v., food vacuole ; v., contractile vacuole ; ps., pseudopodium. Protozoa, and there is also a single contractile vacuole. The endoplasm forms the central mass. It is not vacuo- lated, and contains the large, centrally placed nucleus. Life-history. — An Actinophrys may withdraw its pseudopodia and divide into two,^ with or without the formation of a cyst. A number of individuals may unite for a time by the ectoplasm alone, and separate without any nuclear fusion having taken place (plastogamy). But Schaudinn has described a true sexual process which offers an interesting analogy to the processes of maturation and fertilisation in the higher animals. A number of individuals become joined up in a common 8 114 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS gelatinous cyst. Each loses its pseudopodia and forms a membranous cyst. These cysts become associated in pairs. The nucleus of each cyst divides mitotically and a polar body is extruded from each, after which the nucleus returns to the resting condition. The cysts now fuse in pairs, with complete and intimate union of their nuclei and cell-bodies. The zygote so formed rests for a short period, then divides up into two daughter cysts from which emerge two new individuals of Actinophrys. In the allied genus Actinosphceriiim, with very numerous nuclei, there is a strange and complicated formation and fusion of cysts within a single individual. Third Type of Protozoa — POLYSTOMELLA Polystomella (see Fig. 48) is a type of Foraminifera with a calcareous perforate shell or test. Description. — Polysto- mella crispa is common on the shore, especially among Zostera. It looks like a miniature of an Ammonite shell, and Foraminifera were indeed classified by the older naturalists with the Ammonites. The test forms a close spiral with beautifully chiselled surface ; only the last whorl is visible from the outside. The test is made up of a series of chambers which com- municate with one another and with the exterior by fine pores. Granular protoplasm fills up the chambers and forms also a thin layer on the outside. Long slender pseudopodia issue from the openings in the test and are given off also by the external protoplasmic layer. They frequently branch and anastomose with one another, and their granular protoplasm exhibits marked streaming movements. The pseudopodia serve to catch and entangle the diatoms and Infusoria on which the Foraminifer feeds. Fig. 48. — Polystomella, megalo- spheric form, with large central chamber (M.) and one nucleus (iV.).— After. Lister. POLYSTOMELLA 115 Like many other Foraminifera, Polystomella shows a remarkable dimorphism. It occurs in two forms, outwardly indistinguishable, but differing in internal struc- ture. In the megalospheric form the central chamber is large (a megalosphere), and there is a single large nucleus, placed about the middle of the series of chambers ; in the microspheric form the central chamber is small (a microsphere), being about one-tenth of the diameter of the megalosphere, and there are numerous small nuclei. The megalospheric individuals are about thirty times as numerous as the microspheric individuals. Fig. 49. — Polystomella, microspheric form, with small central chamber {c.c), numerous nuclei {N.), bridges of protoplasm between chambers {B.). — After Lister. Life-history. — The microspheric form has its nuclei replaced by chromidia (chromatin bodies detached from the nuclei into the protoplasm). These chromidia form the centres of amoeboid nucleated spores which leave the shell or are liberated by the protoplasm creeping out and forming a halo of anastomosing threads round the deserted test. The spores secrete a shell and grow into the typical megalospheric forms. When the megalospheric form is about to reproduce, its nucleus disintegrates and is replaced by numerous scattered nuclei formed around chromidia. The proto- Il6 PHYLUM PROTOZOA^THE SIMPLEST ANIMALS plasm segregates into little masses, each centred in a nucleus. Each of these nuclei divides by mitosis into two, then into four, and the division of the nucleus is followed by the division of the protoplasmic mass, so that hosts of tiny cells are formed. These become provided with flagella, swim out into the water, leaving behind them the empty test, and there conjugate in pairs, not with one another but with similar " gametes " from another megalo- spheric individual. The " zygote " so formed becomes the initial chamber of a microspheric individual. In a more direct way — by fission — the megalospheric individual may give rise to another like itself. There is therefore in this complex life-history of Polystomella an alternation between a sexual and an asexual generation. Fourth Type of Protozoa — Paramcecium Paramaecium, a type of ciliated Infusorians, especially of those which are uniformly covered with short cilia (Holotricha). Description. — Specimens of Paramoecium may be readily and abundantly obtained by leaving fragments of hay to soak for some days in a glass of water. A few in- dividuals have been lying dormant about the plant ; they revive and multiply with extraordinary rapidity. They are also abundant in most stagnant pools, and are just visible when a test-tube containing them is held between the eye and the light. Their food consists of small vegetable particles. The form is a long oval, with the blunter end in front ; the outer portion of the cell substance is differentiated into a dense rind or cortex, with a delicate external cuticle, perforated by cilia. There is a definite opening, the so- called mouth, which serves for the ingestion of food particles ; and there is also a particular anal spot posterior to the mouth, from which undigested residues are got rid of. The surface is covered with cilia, in regular longitudinal rows ; these serve both for locomotion and for driving food particles towards the mouth. Paramoecium rotates like a rifle bullet as it swims ; its track is not straight, but an open spiral. If it strikes a solid object or enters an PARAMCECIUM 117 unfavourable medium it " reverses " for a short distance, turns on its side, and goes forward at an angle to the original path. Among the ciUa there are small cavities in the cortex, in which lie fine protrusible threads (" tricho- cysts "). These, though parts of a cell, suggest the thread cells of Coelentera, but are probably of the nature of mooring threads effecting attachment to solid objects. AAA Fig. 50. -Paramcecium in longitudinal optical section, and dividing. — After Butschli. C.V., Contractile vacuole ; MY., longitudinal " myophan " striations ; MA., macronucleus ; MI., one of two micronuclei ; F.V., food vacuole ; CA., a canal in the cytoplasm entering the contractile vacuole which is bursthig through the cortex ; TR., trichocysts at the roots of the cilia (CI.) ; MO., " mouth " leading into gullet. In the right-hand figure D.L. is the transverse dividing line ; the dumb- bell-like elongations of the macronucleus (MA.) and micronuclei (MI.) ; P. A., a "potential anus or weak spot," where debris may be got rid of. The cortical layer is contractile, an^i is distinctly fibrillated. In the substance of the cell lie two nuclei, the smaller " micronucleus " lying by the side of the larger " macro- nucleus." Food vacuoles occur as in the Amoeba, and the digestive process appears to be similar ; but Paramcecium is remarkable for the strength of the acid which it secretes into the vacuoles. There are two contractile vacuoles, from which fine canals radiate into the surrounding proto- ii8 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS plasm ; these discharge into the vacuole, which then bursts to the exterior. Life-history.— Growth is followed by obliquely transverse division into two (Fig. 50, D.L.). One half includes the "mouth," the other has to make one. As well as this simple fission, a process of transient Pjq t^i. — Conjugation of Paramcecium aurelia- stages. — After Maupas. -four Shows macronucleus (A^.) and two micronuclei (m.) in each of the two conjugates. Shows breaking up of macronucleus, and multiplication of micronuclei to eight. Shows the fertilisation in progress ; the macronucleus is vanishing. Shows a single (fertilised) niicronucleus in each conjugate. conjugation also occurs. Two individuals approach one another closely, the two nuclei of each break up, an exchange of pieces of the micro- nucleus takes place ; the two then separate, each to reconstruct its two nuclei (Fig. 51). This process is necessary for the continued health of the species. Fig. 52. — Diagrammatic expression of process of conjugation in Paramcecium aurelia. — After Maupas. The two micronuclei enlarge. Each divides into two. Eight micronuclei are formed. Seven disappear ; one (darkened) divides into two. An interchange and fusion occurs, and the con- jugates separate. The fertilised micronucleus divides into two. Each conjugate begins to divide, the micronucleus of each half dividing into two, one of which becomes the macronucleus, while the others form the two normal micronuclei. The top line repre- sents four individuals, each with a macronucleus and two micronuclei. JLJL ' ' X 1. 1 fl IJ T 4 ' A. B. C. D. E. — ivWv/W/\\ F. / The details of the conjugating process have been worked out with great care by Maupas and others. They differ slightly in different species ; what occurs in P. aurelia is summarised diagrammatically in Fig. 52. The micronuclear elements are represented by two minute bodies. As conjugation begins, these separate themselves from the macronucleus. The macronucleus degenerates, and each micronucleus increases in VORTICELLA II9 size (A). Each divides into two (B) ; another division raises their number to eight (C) ; seven of these seem to be absorbed and disappear, the remaining eighth divides again into what may be called the male and female elements (D) ; for mutual fertilisation now occurs (E). After this exchange has been accomplished, the Infusorians separate, and nuclear reconstruction begins. The fertilised micronucleus divides into two (F), and each half divides again (G), so that there are four in each cell. Two of these form the macronuclei of the two daughter-cells into which the Infusorian proceeds to divide (H) ; the other two form the micronuclei, but before another division occurs each has again divided. Thus each daughter-cell contains a rhacronucleus and two micronuclei. In a "pure line," all descended from one, there is no conjugation. But there is a periodic, usually iflohthlry,: occurrence, as Woodruff and Erdmann have shown, of a reriiarkable p/rocess called endomixis. The nuclei break down as if thei;e.;was _goiDjg to be con- jugation, and then there is re-organisation. ^V >!J- -'/^^ Fifth Type of Protozoa — Vorticella Vorticella, or the bell-animalcule, is a type of those ciliated Infusorians in which the cilia are restricted to a region round the mouth (Peritricha). ,>'| Description. — Groups of Vorticella,^ or of the compound form Carchesium, grow on the stenis of fresh-water plants, and are sometimes readily visible to the unaided eye as white fringes. In Vorticella each individual suggests an inverted bell with a long flexible handle. The base of the stalk is moored to the water-weed, the bell, swings in the water, now jerking out to the full length of its tether, and again cowering down with the stalk contracted into a close and delicate spiral. In Carchesium the stalk is branched, and each branch terminates in a bell. Up the stalk there runs, in a slightly wavy curve, a contractile filament, which, in shortening, gives the non-contractile sheath a spiral form. This contractile filament, under a high power, may exhibit a fine striation. (A similar striated structure is seen in some Amoebae, Gregarines, spermatozoa, etc., and of a much coarser type in striped muscle fibres. It seems to be some structural adaptation to contractility.) The bell has a thickened margin, and within this lies a disc-like lid ; in a depression on the left side, between the margin and the disc, there is an opening, the mouth, which leads by a distinct passage into the cell. On the side of this passage there is a weak spot, the potential anus, by which useless 120 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS debris is passed out. The cilia are arranged so as to waft food particles into the mouth and down the passage. There is a large and horseshoe-shaped macronucleus, and a small micronucleus. Food vacuoles and contractile vacuoles are present as usual. Sometimes a Vorticella bell jerks itself off its stalk and swims about ; in other conditions it may form a temporary -nig- 4 5- Fig. 53. — Vorticella. — After Biitschli. Structure. A'^., Macronucleus ; «., micronucleus ; c.v., con- tractile vacuole ; m., mouth ; f.v., food vacuole ; v., vestibule. Encysted individual. 3. Division. Separation of a free-swimming unit — -the result of a division. Formation of eight minute units (tng.). 6. Conjugation of microzooid (mg.) with one of normal size. cyst ", normally, the cilia are very active, and the move- ments of the stalk frequent and rapid. Multiplication may take place by longitudinal fission — a bell divides into similar halves ; one of these acquires a basal circlet of cilia and goes free, ultimately becoming fixed. Or the division may be unequal, and a microzooid, or as many as eight, may be set free. These swim away by means of the posterior girdle of cilia, and each may conjugate with an VOLVOX 121 individual of normal size. In this case a small active cell (like a spermatozoon) fuses intimately with a larger passive cell, which may be compared to an ovum. Sixth Type of Protozoa — Volvox Volvox is a type of flagellate Infusorians, especially of those with flagella of equal size. Volvox is found, not very commonly, in fresh-water pools, Fig. 54. — Volvox globator. — After Klein and Janet. I. and v. Biflagellate individuals. II. Ripe ovum. III. A ball of sperms. IV. A daughter colony developing. and is usually classed by botanists^ as a green Alga. It consists of numerous biflagellate individuals, connected by fine protoplasmic bridges, and embedded in a gelatinous matrix, from which their flagella project, the whole forming a hollow, spherical, actively motile colony. In V. globator the average number of individuals is about 10,000 ; in V. aureus or minor, 500-1000. The individual cells are stellate or amoeboid in V. globator^ more spherical in V. 122 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS aureus ; each contains a nucleus and a contractile vacuole. At the anterior hyaline end, where the flagella are inserted, there is a pigment spot ; the rest of the cell is green, owing to the presence of chlorophyll corpuscles. In consequence of the presence of these, Volvox is holophytic, i.e. it feeds as a plant does and builds up starch granules. In its method of reproduction Volvox is of much biological interest and importance. As Klein, one of its best describers, says, it is an epitome of the evolution of sex. Some of the colonies are asexual. In these a limited number of cells possess the power of dividing up to form little clusters of cells ; these clusters escape from the envelope of the parent colony, and form new free-swimming colonies. In other colonies there are special reproductive cells, which may be called ova and spermatozoa. In V. globator the two kinds of reproductive cells are usually formed in the same colony, the formation of spermatozoa generally preceding that of the ova. Technically the colony may then be described as a protandrous hermaphrodite. In V. aureus the colony is oftenest unisexual or dioecious, i.e. either male or female. But it may be monoecious or hermaphrodite, and is then generally protogynous, i.e. producing eggs first. Whether in a hermaphrodite or in a unisexual colony, the sex cells appear among the ordinary vegetative units ; the ova are distinguishable by their larger size, the " sperm mother cells " divide rapidly and form numerous (32-100 or more) slender spermatozoa, each with two cilia. In V. globator their bundles may break up within the parent colony ; or, as always occurs in V. aureus, they may escape intact, and swim about in the water. In any case, an ovum is fertilised by a spermato- zoon, and, after a period of encystation and rest, segments to form a new colony. Occasionally, however, this organism, so remarkable a condensation of reproductive possibiUties, may produce ova which develop parthenogenetically. Here, then, we have an organism, on the border line between plant and animal life, just across the line which separates the unicellular from the multicellular, illustrating the beginning of that important distinc- tion between somatic or body cells and reproductive cells, and occurring in asexual, hermaphrodite, and unisexual phases. Klein records no less than twenty-four different forms of V. aureus from the purely vegetative and asexual to the parthenogenetic, for there may be almost entirely male colonies, almost entirely female colonies, and other interesting transitional stages. Klein has also succeeded to some extent in showing that the occurrence of the various reproductive types depends on outside influences. Seventh Type of Protozoa — Monocystis Monocystis, a type of Sporozoa in which the cell is not divided into two parts by a partition. MONOCYSTIS 123 Description. — Two species (M. agilis and M. magna) infest the male reproductive organs of the earthworm. The full-grown adults are visible to the naked eye — flattened worm-like cells ; the shape alters during the sluggish move- ments. Peripherally there is a porous cuticle, a clear cortical zone, and a network of myoneme fibrils. In a more fluid medullary substance, the large nucleus floats. There are numerous granules of protein, carbohydrate, and other sub- stances. In one species there is an anterior projection like the cap of Gregarina, otherwise unrepresented in Monocystis. As in Gregarina, and many other parasitic forms, a con- tractile vacuole is absent. Life-history. — The young form of M. agilis is parasitic sp.c Fig. 55. — Life-history of Monocystis. — After BUtschli. 1. Young Gregarine lying within a sperm mother cell of earthworm. 2. Association of two Gregarines within a cyst, ready to form gametes. 3. Numerous spore-cases (sp.c, pseudonavicellae) within a cyst. 4. A spore-case with eight spores (sp.) and a residual core {rb.). * within one of the sperm mother cells of the earthworm. It grows, and becomes free from the cell as a trophozoite. In the free stage, two individuals may unite in a curious end-to-end manner observed also in Gregarina. Quite diflFerent is the association of two individuals (gametocytes) inside a common cyst. After a process of " reduction " the nucleus of each divides repeatedly, and the daughter nuclei migrate to the surface of the cell, where each is surrounded by a little mass of protoplasm. Each of the gametocytes thus gives rise to a number of gametes ; there remains a mass of residual protoplasm. The wall between the two gametocytes now breaks down and the gametes conjugate in pairs, forming zygotes . In each pair of conjugating gametes one is probably derived from each gametocyte. Each zygote secretes a membrane and becomes a spore-case. The 124 PHYLUM PROTOZOA^ — THE SIMPLEST ANIMALS nucleus divides up, and eight elongated spores are formed round a residual core. The spore-case now takes its typical shape and is known as a pseudonavicella. The spores are considerably larger than those of Gregarina. Eventually, in the alimentary canal of another earthworm the cyst bursts, the spore-cases are extruded, the spores emerge from their firm chitinoid cases. The young spore (sporozoite) is like a bent spindle (falciform), and seems next door to being flagellate. It bores into a mother sperm cell, and from this Fig. 56. — Life-history of Gregarina. — After Butschli. 1. Young forms {a, b, c) emerging from intestinal cells (i.e.) ; i.n., nucleus of intestinal cell. 2. Two forms conjugating (0. blattarum). 3. Spore formation within a cyst. 4. .\dult with deciduous head-cap (c.c), and a cuticular partition dividing the cell into an anterior part (A ) and a posterior part (B) ; n., the nucleus. 5. A spore-case (sp.c). it afterwards passes as an adult into the cavity of the seminal vesicles. Intracellular parasitism and copious food naturally act as checks to activity, and the adult is sluggish. The allies of Monocystis occur chiefly in " Worms," Tunicates, and x\rthropods ; none are known in Vertebrates. Along with Monocystis we take Gregarina, a type of Sporozoa in which the cell is divided into two regions by a partition. Various species occur in the intestine of the lobster, cockroach, and other Arthropods. When young they are intracellular parasites, but later they become free in PLASMODIUM 125 the gut. They feed by absorbing diffusible food-stuffs, such as peptones and carbohydrates, from their hosts, and store up glycogen within themselves. In many the size is about one-tenth of an inch. There is a firm cuticle of " proto- elastin," which grows inwards so as to divide the cell into a larger nucleated posterior region and a smaller anterior region, and also, in the young stage, forms a small anterior cap. The cell substance is divided into a firmer cortical layer and a more fluid central substance. The protoplasm often presents a delicate fibrillar appearance, suggesting that of striated muscle. The nucleus is very dis- tinct, but there are no vacuoles. We may associate the absence of locomotor pro- cesses, " mouth," and contractile vacuoles, as well as the thickness of the cuticle and the general passivity, with the parasitic habit of the Gregarines. The young Gregarine is parasitic in one of the hning cells of the gut ; it grows, and, leaving the cell, remains for a time still attached to it by the cap (Fig. 56, «, b, c) ; later this is cast off, and the individual be- comes free in the gut, w^hile still increasing in size. Two or more individuals attach themselves together end to end, but the meaning of this is obscure. Encystation occurs, involving a single unit or two to- gether. The details of spore-formation are similar to those in Monocystis. Eventually the cyst bursts, the spore-cases are liber- ated, and from within each of these eight spores emerge to become cellular parasites. G. {Porospora) gigantea is sometimes three-quarters of an inch in length — enormous for a Protozoon. to Fig. 57. — End- to-end union of Gregarines. — After Frenzel. The adult of Eighth Type of Protozoa — Plasmodium vivax Plasmodium, one of the Haemosporidia, is parasitic in the red blood cells of man and other Vertebrates. P. vivax benign tertian " malaria in man. The Hfe- causes 126 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS history falls into two parts : (i) asexual or schizogony stage, passed in man ; (2) sexual or sporogony stage, passed in a mosquito belonging to the genus Anopheles. Life-history. — (i) The Plasmodium enters its verte- brate host as a minute and very slender spindle-shaped creature in the saliva which mosquitoes inject when they bite. It is then in its sporozoite stage. It burrows into a red blood cell and gradually comes to rest within it. This is the trophozoite stage. The Plasmodium becomes a rounded body with a single large nucleus. A large vacuole frequently develops, pushing the nucleus to one side and giving a characteristic ring-like appearance (" signet-ring form "). At the conclusion of the trophozoite period the parasite enters on the schizont stage. Multiple fission of the nucleus takes place, and the cytoplasm, save for a small residual amount containing pigment granules, aggregates round the daughter nuclei to form 15-20 merozoites. The red blood cell then bursts, and the merozoites are set free in the blood. The merozoites attack fresh red blood cells, and the cycle, which takes forty-eight hours, is repeated. After about five cycles have been passed, the amount of toxin liberated into the blood from the shattering of the red blood cells sets up fever in the human host, and attacks recur every forty- eight hours, corresponding with the escape of the mero- zoites. Eventually the second stage in the life-history becomes inaugurated. Certain merozoites grow more slowly than the others, and do not produce schizonts, but, entering a red blood cell, become rounded oflF without developing a vacuole, and after reaching their limit of growth, become free in the blood as male and female gametocytes, the latter somewhat larger in size than the former,"but with a smaller nucleus. (2) If at this stage some of the gametocyte-containing blood is sucked in by an anopheline mosquito, the gametocytes alone survive digestion, and sporogony begins. The male gametocytes give rise to 4-8 slender active microgametes, which swim rapidly about until contact occurs with a macrogamete produced after maturation from a female gametocyte. The tiny microgamete enters the macrogamete, their nuclei fuse, and a zygote (fertilised cell) is formed. The COCCIDIUM 127 zygote becomes surrounded with a thin pelUcle, becomes pointed at both ends, and works its way into the m.osquito's gut wall, where it burrows through the lining and comes to rest, developing a globular envelope or oocyst. Great growth now takes place, both of oocyst and contents. Ultimately there are formed within the oocyst an enormous number of elongated sporozoites. The oocyst then bursts, and the sporozoites are set free in the mosquito's blood spaces. Very many of them accumulate in the salivary glands, and are passed out with the salivary juice to start the life-cycle anew. Ninth Type of Protozoa — Coccidium schubergi Reference may here be made to the common Coccidia, intracellular Sporozoa, attacking mainly the epithelial cells of the gut or associated organs. They are found chiefly in insects, myriopods, molluscs, and vertebrates. Thus Coccidium schubergi infests the intestinal epithelium of the centipede Lithobius forficatus. The adult is a minute oval or spherical cell with a nucleus. It lives a quiescent life within the host cell, growing and absorbing nourish- ment until the resources of the cell are exhausted. Life-history. — The coccidium enters the host cell as a minute sickle-shaped body, pointed at the anterior end, and more blunt posteriorly. This is the sporozoite stage of the life-history ; it is liberated from a cyst (oocyst) when the latter is swallowed by the centipede in its food. When freed in the gut the sporozoite progresses by forward gliding movements, alternating these by flexions, bending itself like a bow and straightening out again. When about to enter an epithelial cell it presses the anterior end through the cell wall and wriggles its way in. Once within the cell in which development is to proceed, its movements gradually cease, but it may pass through several cells before coming to rest. Within the host cell the coccidium — now in the trophozoite stage — becomes oval in form, and in about twenty-four hours has reached full size and has exhausted the host cell contents. This is the completion of the trophozoite period, and the parasite now enters the schizont stage, where its nucleus divides into a number of daughter nuclei. These 128 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS arrange themselves around the periphery of the cell, whilst the protoplasm breaks up to form along with them bodies of a shape similar to the sporozoites. There are important structural differences, however, apart from the difference in origin. The parasites, now known as merozoiteSy rupture Fig. 58. — Life-history of Coccidium. — After Schaudinn. I. Sporozoite ; 2. Sporozoite entering a cell and becoming a trophozoite; 3-4. Schizont, forming merozoites ; 5. Merozoites entering another cell ; 6a. Merozoite forming macrogamete ; 6b. Merozoite forming microgametes; 7. Free microgamete ; 8-9. Fertilisation of macrogamete by microgamete ; 10. Zygote within oocyst ; 11. Formation of spores within oocyst ; 12. Spores forming sporozoites. the host cell, move in the gut cavity after the manner of the sporozoites, enter fresh epithehal cells, and repeat the fore- going cycle until ultimately the greater part of the gut epithelium is destroyed. In about five days, however, owing perhaps to the failing capacity of the host to nourish, the limit of asexual reproductivity is reached, and the Classes of protozoa 129 parasite now enters upon a spore-forming stage. Certain merozoites grow more slowly than the others, and instead of becoming schizonts give rise to elements of two types, viz. microgametes, slender cells bearing a flagellum at each end, which are male, and macrogametes, larger bean-shaped cells, which are female. The latter after maturation free themselves from the host cell, and in the cavity of the gut are fertilised by a male element. After fertilisation, a trans- parent membrane forms around the zygote (fertilised cell). This membrane in the first instance serves to exclude all microgametes after the first, and later, becoming very tough and resistant, forms a protecting envelope or oocyst. After the oocyst is formed the parasite may pass from the host to the exterior or remain for some time longer within it. The nucleus of the zygote within the oocyst now divides into four, around which the protoplasm aggregates itself to form the spores. There are thus four spores within a cyst. Each spore divides, forming two sporo.zottes , which on the arrival of the oocyst in the gut of a fresh host are liberated, and attacking the lining epithelium recommence the life- history. General Classification of Protozoa Since the Protozoa are unicellular organisms (except the few which form loose colonies), their classification should be harmonious with that of the cells in a higher animal. This is so. Thus (a) the Rhizopods, in which the living matter flows out in changeful threads or " pseudopodia," as in the common Amoeba^ are comparable with the white blood corpuscles or leucocytes, many young ova, and other " amoeboid " cells of higher animals ; (b) the Infusorians, which have a definite rind and bear motile lashes (cilia or flagella), e.g. the common Paramoecium^ may be likened to the cells of ciliated epithelium, or to the active sperma- tozoa of higher animals ; {c) the parasitic Sporozoa, which have a rind and no motile processes or outflowings, may be compared to degenerate muscle cells, or to mature ova, or to " encysted " passive cells in higher animals. This comparison has been worked out by Professor Geddes, who also points out that the classification represents the three physiological 130 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS ■possibilities: (a) the amoeboid units, neither very active nor very passive, form a median compromise ; {b) the ciHated Infusorians, which are usually smaller, show the result of a relative predominance of ex- penditure ; (c) the encysted Gregarines represent an extreme of sluggish passivity. But, as Geddes and others have shown, the cells of a higher animal often pass from one phase to another — the young amoeboid ovum accumulating yolk becomes encysted, the ciliated cells of the windpipe may, to our discomfort, sink into amoeboid forms. The same is true of CORTICATA.) Predominantly ciliated and active. Infusorians. Classification of Protozoa (GVMNOMYXA.) Predominantly amoeboid. Rhizopods. (CORTICATA.) Predominantly encysted and passive. Sporozoa. ACINETARIA. RADIOLARIA. CILTATA. Rhynchoflagellata. DiNOFLAGELLATA. FLAGELLATA. FORAMINIFERA. Labyrinthulidea. Heliozoa. LOBOSA. SPOROZOA OR GREGARINES. Proteomyxa and Mycetozoa. Primitive Forms. the Protozoa ; thus in various conditions the ciliated or flagellate unit may become encysted or amoeboid, while in some of the simplest forms, such as Proiomyxa, there is a " cell-cycle " in which all the phases occur in one life-history. Systematic Survey A. Primitive forms.— Under this heading may be included two classes : I ) the Proteomyxa, primitive, insufficiently known forms often without a nucleus, though nuclear material may be present in the form CLASSES OF PROTOZOA I3I of scattered granules (chromidia), and (2) the Mycetozoa, organisms with somewhat complex fructifications, often classed as plants allied to Fungi. As examples of the Proteomyxa, we have the interesting Protomyxa in four phases : (a) encysted and breaking up into spores, which (6) are briefly flagellate, (c) sink into amoeboid forms, and {d) flow together into a composite " Plasmodium " ; Vampyrella, parasitic on fresh -water Algae ; and many others. The Mycetozoa are well illustrated by Fuligo or yEtkalium septicum, " flowers of tan," found in summer as a large plasmodium on the bark of the tan-yard. The coated spores are formed in little capsules which rise from the surface of the plasmodium. The spores may be first flagellate, then amoeboid, or amoeboid from the first ; the characteristic Plasmodium is formed by the fusion of the amoebae. B. Predominantly amoeboid Protozoa. — Rhizopoda. — The simplest Rhizopods generally resemble Amoeba, and are ranked in the class (3) Lobosa. They may reproduce simply by division, as does Amceba itself, or mav liberate several buds at once (Arcella), or form Fig. 59. — Diagram of Protomyxa aurantiaca. — After Haeckel. I. Encysted ; 2. Dividing into spores ; 3. Escape of spores, at first flagellate, then amoeboid ; 4. Plasmodium, formed from fusion of small amoebae. spores which conjugate {Pelomyxa). Various forms, siich as Arcella, are furnished with a shell. (4) The I.abyrinthulidea are represented by forms like Labyrinthula on Algae, and Chlamydomyxa on bog-moss, which consist of a mass of protoplasm spread out into a network, and of numerous spindle-shaped units, which travel continually up and down the threads of the living net. As (5) Heliozoa are classified the sun-animalcules [Actinosphcsrium, Actinophrys sol), and others, in which there are stiff processes radiating from a spherical body. Reproduction may be by division or by spore formation ; skeletal structures may be represented by spicules. The (6) Foraminifera or Reticularia include an interesting series of shelled forms in which the peripheral protoplasm forms branching interlacing threads. A few simple forms occur in fresh water ; the great majority occur on the floor of the sea at varying depths ; some families are abundantly represented on the surface. The shell is usually calcareous, more rarely arenaceous or chitinous. There is sometimes dimorphism. Multiplication occurs by fission, or by the formation of swarm-spores (amoeboid or flagellate). Foraminifera are common as fossils from Silurian rocks onwards, and at the present day are very important in the formation of calcareous ooze ; in this respect Globigerina, with a chambered shell, is especially important. Species 132 PHYLUM PROTOZOA— THE SIMPLEST ANIMALS of Gromia are found in both fresh and salt water ; Haliphysema, a form utilising sponge-spicules to cover itself, was once mistaken for a minute sponge. Most kinds of chalk consist mainly of the shells of Foraminifera accumulated on the floor of ancient seas ; Nummulites (Fig. 17) and related fossil forms were as large as shillings or half-crowns. More complex are the (7) Radiolaria, which are divided by a chitinoid membrane into an inner central capsule (with one or more nuclei), and Fig. 60. — Formation of shell in a simple Foraminifer. — After Dreyer. In A and B the shell has one chamber ; C and D show the formation of a second. Note outflowhig pseudopodia and the enclosure of the shell by a thm layer of protoplasm ; note also the nucleus in the central protoplasm. an outer portion, gelatinous and vacuolated, giving off radiating thread- like pseudopodia, which very rarely interlace. There is usually a skeleton in the form of a siliceous lattice-work or regularly disposed spicules outside the central capsule, but in some cases the shell is formed of a horn-like substance called acanthin, which is probably a complex silicate. Radiolarians multiply by fission, which sometimes includes a halving of the skeleton, and by spores, which in some cases are dimorphic. Most Radiolarians include unicellular Algae (yellow cells), with which they live in intimate mutual partnership (symbiosis). Most Radiolarians float on the surface of the sea ; others live below CLASSES OF PROTOZOA 133 the surface at varying depths ; and some are abyssal. They are abundant as fossils,' and of much importance in the formation of the ooze of great depths. • , ^ „ Examples.— Thalassicola, EucyrUdtum, and the colonial Collozoum and Sphcerozoum. ,1 , v C. Predominantly active forms (ciliate and flagellate), Fig. 61.— a pelagic Foraminifer — Hastigerina {Globigerina) murrayi. — After Brady. Note central shell, projecting calcareous spines with a protoplasmic axis ; also fine curved pseudopodia and vacuolated protoplasm. generally called Infusorians.— Protozoa, with a definite rind and with 1-3 undulating flagella, aie included as (8) Flagellata, a very large group, among which are such familiar forms as the common Euglena of ponds ; the Monads ; Volvox, a colonial form ; Codosiga, a colony in which the individual cells are furnished with a collar (Choano- flagellata). The Ha^moflagellata are important blood parasites, gener- ally called Trvpanosomes (see p. i47)- Modified flagellate forms are included in the groups Dmoflagellata 134 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS and Cystoflagellata, in both of which there are two flagella, differ- ently placed in the two cases. In the first are included Peridinium and Fig. 62. — The trumpet -shaped cihated Infusorian, Stentor. — After Stein. A^., Beaded iiiacronucleus ; «., divided niicronucleus ; C.V., contractile vacuole ; AD., large adoral cilia, spirally arranged ; C7., small cilia all over ; P., a particle being wafted into the mouth, which is near the centre of the whirlpool. Numerous food vacuoles are seen in the general cytoplasm. Cerattum ; in the latter, the large phosphorescent Noctiluca. form an important part of the plankton of lakes and sea. They CLASSES OF PROTOZOA 1 35 As (9) Ciliata are included a very large number of forms, more or less closely resembling Paramcecium or Vorticella, and very abundant in infusions ; some, such as Opalina, in the intestine of the frog, are more or less parasitic. As specially modified Ciliata are included (10) Acinetaria, highly specialised forms, ciliated when young, but usually fmrnished when adult with suctorial tentacles. They are fixed in adult life, and feed on other Protozoa. As examples may be given Acineta ; Dendrosoma, forming branched colonies ; and Ophryodendron, without suctorial tentacles. Some, like Sphcerophrya, are minute and parasitic. D. Predominantly encysted Protozoa. — Sporozoa. — Forms like Gregarina and Monocystis are included in a group of the (11) Fig. 63. — Optical section of a Radiolarian (Actinomnia). —After Haeckel. a Nucleus ; b, wall of central capsule ; c, siliceous shell within 'nucleus; c', middle shell within central capsule; c2, outer shell in extra-capsuleir substance. Four radial spicules hold the three spherical shells together. Sporozoa, the Gregarinida in the strict sense. They are parasites in the gut or body cavity of many Invertebrates, especially Arthropods. Coccidiiun is a type of the Coccidiidea, which are intracellular parasites occurring in Arthropods, Molluscs, and Vertebrates. A very im- portant group, with a life-cycle essentially similar to that of the Coccidiidea, are the Haemosporidia, which are parasitic in the red blood corpuscles of Vertebrates. The malaria parasites belong to this group. In many of the Haemosporidia a part of the hfe-cycle takes place in an intermediate host, usually a mosquito or a tick. Other groups of the Sporozoa are the Myxosporidia, with peculiar nematocyst-hke organs (Invertebrates and cold-blooded Vertebrates), and the Sarcosporidia, which are found inside the striped muscles of warm-blooded Vertebrates. 136 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS General Notes on the Functions of Protozoa Movement. — The simplest form of movement is that termed amoeboid, as illustrated by an Amoeba. In ordinary conditions it is continually changing its shape, putting forth blunt lobes and drawing others in. Surface tension phenomena occur on the outermost zone of the cytoplasm, and also beneath that — often along the dorsal surface in the direction of motion, then over the front end, then along the ventral surface next the substratum, then again at the posterior end dorsally. Thus there is a complex " caterpillar- wheel "-like stream.ing movement of the granules. No final explanation of the whole process in physico-chemical terms has yet been given. A more defined contraction, like that of a muscle cell, is illustrated in the contractile filam.ent of the stalk of Vorticella and similar Infusorians ; and not less definite are the move- ments of cilia and flagella, by means of which most In- fusorians travel swiftly through the water. Cilia in movement are bent and straightened alternately ; while flagella, which are usually single mobile threads, exhibit lashing movements to and fro, or, more often, are held stretched out in front, and by a curious rotatory movement draw the cell along. They are then more aptly termed tractella. It seems probable that cilia and flagella consist of an elastic core surrounded by a sheath, which may be uniformly contractile, or may have one contractile band, or two opposite contractile bands, and so on. Considered generally, the movements are of two kinds : either (i) re- flex, i.e. responses to external stimulus, as when the Protozoon moves towards a nutritive substance ; or (2) automatic, i.e. such movements as appear to originate from within, without our being able to point to the immediate stimulus, e.g. the rhythmical pulsations of contractile vacuoles. Actively moving Protozoa usually show the following motor reaction to stimulus : — they move backward, turn over on one side structurally defined, and then move forward again. Sensitiveness. — The Amoeba is sensitive to external in- fluences. It shrinks from strong light and obnoxious materials ; it moves towards nutritive substances. This sensitiveness is, so far as we know, diffuse — a property of PHYSIOLOGY OF PROTOZOA 1 37 he whole of the cell substance ; but the pigment spots of some forms are specialised regions. Many Protozoa well illustrate a strange sensitiveness to the physical and chemical stimuli of objects or substances with which they are not in contact. Thus the simple amoeboid Vampyrella will, from a con- siderable distance, creep directly towards the nutritive substance of an Alga, and the Plasmodium of a Myxomycete will move towards a decoction of dead leaves, and away from a solution of salt. The same sensitiveness, technically termed chemotaxis, is seen when micro- organisms move towards nutritive media or away from others, when the spermatozoon (of plant or animal) seeks the ovum, or when the phago- cytes (wandering amoeboid cells) of a Metazoon crowd towards an in- truding parasite or some irritant particle. Nutrition. — The Amoeba expends energy as it lives and moves ; it regains energy by eating and digesting food particles. Most of the free Protozoa live in this manner upon solid food particles ; a few, such as Volvox, in virtue of their chlorophyll, are holophytic, i.e. they feed like plants ; the parasitic forms usually absorb soluble and diffusible substances from their hosts. Respiration. — Oxygen is simply taken up by the general protoplasm from the surrounding medium, into which the waste carbonic acid is again passed. The bubbles which enter with the food particles assist in respiration. In parasitic forms the method of respiration must be the same as that of the tissue cells of the host. Excretion. — Of the details of this process little is certainly known, but the contractile vacuoles are, without doubt, primitive excretory appliances. In the more specialised forms they appear to drain the cell substance by means of fine radiating canals, and then to burst to the exterior. Uric acid and urates are said to be demonstrable as waste products. Colour. — Pigments are not infrequently present in the Protozoa. We have already noticed the presence of chlorophyll in some forms ; with Radiolarians the so-called " yellow cells " are found almost constantly associated. Each of these cells consists of protoplasm, surrounded by a cell wall, and containing a nucleus. The protoplasm is impregnated with chlorophyll, the green colour of which is obscured by a yellow pigment. Starch is also present. The cells multiply by fission, and continue to live after isolation from the protoplasm of the Radiolarian. All these facts point to the conclusion that the cells are symbiotic Algae, so-called Zoochlorellce. According to some, the " chlorophyll corpuscles " seen in the primitive Archerina, in some 138 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS flagellate forms, as Euglena, and in many Ciliata, as Stentor, Stylo nichia, one species of Paramcecium, Volvox and the allied forms, are also symbiotic Alga), which have lost the power of independent exist- ence. The evidence for this is, however, insufficient, and this explana- COL ■ Fig. 64. — A Monad Infusorian. — After Saville Kent. N., Nucleus; C.V., contractile vacuole; F.V., food vacuole ; F.P., an external food particle ; the arrows indicate how the particles are swept ; COL., trans- parent collar in the middle of which the flagcUum (FL.) works. tion will not apply in cases like that of VorticeUa viridis, where the green colouring matter is uniformly distributed through the protoplasm. In many cases there is, besides the chlorophyll, a brown pigment, identical with the diatomin of Diatoms. In many of the Flagellata there are one or more bright pigment spots at the anterior end of the STRUCTURE OF PROTOZOA 1 39 cell ; these may be specially sensitive areas. . In some of the simpler Gregarines the medullary protoplasm is coloured with pigment which is apparently a derivative of the hsemoglobin of the host. Psychical life. — Protozoa often behave in a way which suggests control, but it should be noted that cut-off frag'ments sometimes behave just as effectively as the intact units. Verworn has decided, after much labour, that the Protozoa do not exhibit what even the most generous could call intelligence ; but this is no reason why he or any other evolutionist should doubt that they have in them the indefinable rudiments of mind. Jennings has shown that the behaviour of some Infusorians corresponds to what may be called the method of trial and error ; they " try " one kind of response after another until, in some cases, they give the effective answer. General Notes on the Structure of Protozoa The Protozoa are sometimes called " structureless," but they are only so relatively. For though they have not stomachs, hearts, and kidneys, as Ehrenberg supposed, they are not like drops of white of egg. The cell substance consists of a living colloidal mixture, often with vacuoles. In many cases there are numerous granules, some of which are food fragments in process of digestion, or waste products in process of excretion. The cell substance includes one or more nuclei, special- ised bodies which are essential to the life and multiplication of the unit. In the Protozoa there are several conditions under which the nucleus may exist : — (i) In some adult forms, and in many spores or young forms, no definite nucleus has yet been discovered. It is, however, unnecessary to preserve the term " Monera " for such simple forms, as it is probable that nuclear material does exist in the form of granules. (2) In the majority of cases, notably in, the Sporozoa, the nucleus is single, often large, and placed centrally. From a consideration of the cells of Metazoa we may call this the typical case. (3) In many of the Ciliata, e.g. Pararncecium, there are two dimorphic nuclei. There is a large oblong nucleus, and beside it a smaller spherical one. {4) In some Ciliata the macronucleus exists in the form of powder scattered through the protoplasm, e.g. in Opalinopsis. The granules may collect to form a compact nucleus when fission is about to take place. 140 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS (5) In Opalina, from the intestine of the frog, and a few other forms, there are very numerous nuclei, arranged in a symmetrical manner in the cell substance. In some cases these isolated nuclei have been observed to unite to form one large nucleus just before binary fission takes place. Of these various cases the diffuse condition is apparently very primitive. The nucleus, when stained and examined under high powers, is observed to be complex in structure. It consists of a nuclear network, or a coil of chromatin threads. Karyokinesis has been observed in some cases. While we cannot at present define the physiological import of the nucleus, we must recognise its importance. Thus Bruno Hofer has shown that when an Amceha is cut in two, the part with the nucleus lives and grows normally, while the part without any nucleus sooner or later dies ; and Balbiani has observed that in the case of Infusorians cut into pieces, those parts which have nuclei survive, while if no nucleus is present in the fragment, the wound may remain unhealed, and death ensues. The outer part of the cell substance (*' ectoplasm ") is often clearer and less granular than the inner part ('' endo- plasm "). In corticate Protozoa there is a more definite rind or thickened margin of cell substance. Outside this there may be a " cuticle " distinct from the living matter , sometimes consisting of chitin, or gelatin, or rarely of cellulose. The cuticle may form a cyst, which is either a protection during drought, or a sheath within which the unit proceeds to divide into numerous spores. Moreover, the cuticle may become the basis of a shell formed from foreign particles, or made by the animal itself of lime, flint, or organic material. In the cell substance there may be bubbles of water taken in with food particles (food vacuoles), contractile vacuoles, fibres which seem to be specially contractile (in Gregarines), spicules of flint or threads of horn-like material, which may build up a connected framework, and the pigments already mentioned. Some marine and fresh-water Protozoa secrete bubbles of oxygen which buoy them up in the water. Reproduction of Protozoa Growth and reproduction are on a different plane from the other functions. Growth occurs when income exceeds expenditure, and when constructive or anabolic processes REPRODUCTION IN PROTOZOA 141 are in the ascendant. Reproduction occurs at the hmit of growth, or sometimes when nutrition is checked. As it is by cell division that all embryos are formed from the egg, and all growth is effected, the beginnings of this process are of much interest. (a) Some very simple Protozoa seem to reproduce by what looks like the rupture of outlying parts of the cell substance. (&) The production of a small bud from a parent cell is not uncommon, and some Rhizo- pods (e.g. Arcella, Pelomyxa) give off many buds at once, (f) Com- moner, however, is the definite and orderly process by which a unit Fig. 65. — Diagram of the structure of Noctiluca. FL., Large locomotor flagelluin ; fl., small food-inwafting flagellum ; M., mouth ; iV., nucleus ; P., central protoplasm around nucleus, with granules and food- particles ; R., a ridge in the "gullet" — a con- tinuation of the mouth opening ; V.C., much vacuo- lated general cytoplasm ; CU., cuticular pelUcle. divides into two — ordinary cell division, {d) Finally, if many divisions occur in rapid succession or contemporaneously, and usually within a cyst enclosing the parent cell, i.e. in narrowly limited time and space, the result is the formation of a considerable number of small units or spores. In the great majority of cases, each result of division is seen to include part of the parent nucleus. A many-celled animal multiplies in most cases by liberating reproductive cells — ova and spermatozoa- different from the somatic cells which make up the " body." A Protozoon multiphes by dividing wholly into daughter 142 PHYLUM PROTOZOA — tHE SIMPLEST ANIMALS cells. This difference between Metazoa and Protozoa in their modes of multiplication is a consequence of the difference between multicellular and unicellular life. Each part of a divided Protozoon is able to live on, and will itself divide after a time, whereas the liberated spermatozoa and ova of a higher animal die unless they unite. By sexual reproduction we mean — (a) the liberation of special reproductive cells from a " body," and (b) the fertilisation of ova by spermatozoa. As Protozoa have no " body " — though the beginnings of one are seen in the colonial forms — they cannot be said to exhibit sexual reproduction in the first sense (a), yet many of them^ (especially the Sporozoa) give origin by division to special reproductive cells. And although many Protozoa can live on, dividing and nujltiplying, for prolonged periods without the occurrence of anything like fertilisation, processes corresponding to fertilisation are of general occurrence. For in many of the Protozoa there occurs at intervals a process of " conjugation *' in which two individuals unite either permanently or temporarily. This is an incipiently sexual process ; it is the analogue of the fertilisation of an ovum by a spermatozoon. In many cases, moreover, there is a difference between the two conjugates, analogous to the difference between ovum and spermatozoon. (i) It is one of the recurrent phases in the Hfe-history of some of the simplest Protozoa (Proteomyxa and Mycetozoa) (see Fig. 59), that a number of amoeboid units flow together into a composite mass, which has been called a " Plasmodium.'" (2) It is known that more than two individual Sporozoa and other forms occasionally unite. To this the term " multiple conjugation " has been applied. (3) Commonest, however, is the union of two apparently similar individuals, either permanently, so that the two fuse into one, or temporarily, so that an exchange of material is effected. Permanent conjugation has been observed in several Rhizopods, Infusorians, and Sporozoa. Temporary conjugation is well known in not a few ciliated Infusorians, and it is possible that a curious end-to-end union of certain vSporozoa is of the same nature, or it may be of the nature of a " Plasmodium " formation. The formation of small spores (gametes) which conjugate is not uncommon. (4) There are some cases where one of the conjugating individuals is larger and less active than the other. Thus in Vorticella, a small free-swimming form unites and fuses completely with a stalked indivi- dual of normal size. This " dimorphic conjugation " is evidently CONJUGATION 143 analogous to the fertilisation of a passive ovum by an active sper- matozoon. In Volvox this is even more obvious, for the small and active cells, both in shape and method of formation, recall the sper- matozoa of higher forms. Significance of Conjugation. — Conjugation is an episode in repro- duction, not a mode of multiplication. It promotes variability in a stock and it helps to ward off senescence. All the descendants of a single individual form what is called " a pure line," and conjugation does not occur among the members of a pure line. Asexual multiplica- tion may continue in ideal conditions for ten years and through thou- FiG. 66. — Spore-formation in Noctiluca. — After Roule- From minute nucleated hillocks on the surface of the cell peculiar flagellate spores (SP.) are given off. The long flagellum becomes the short oral flagellum of the adult. The peak- like process develops into the large locomotor flagellum of the adult (see Fig. 65). sands of generations. In ideal conditions of experimental isolation there is no falling off in the vigour of the stock. But in such cases, as also in certain species of Paramoecium in natural conditions, there is a periodic, approximately monthly occurrence of a process called by Woodruff and Erdmann " endomixis." There is a nuclear disintegra- tion, followed by reintegration. It is similar to the preliminary stages leading on to conjugation, but no conjugation occurs. Endomixis is a process that assists rejuvenescence. Ecology. — Many Protozoa raise organic debris once more into the circle of life, and many form part of the food 144 PHYLUM PROTOZOA — THE SfMPLEST ANIMALS of higher animals. Thus those pelagic Foraminifera and Radiolarians, which sink dying to the great oceanic depths, form along with more substantial debris the fundamental food supply in that plantless world. Fundamental, since it is plain that the deep-sea animals cannot all be living on one another. Almost every kind of nutritive relation occurs among the Protozoa. Predatory life is well illustrated by most In- fusorians, and thoroughgoing parasitism by the Sporozoa ; Opalina in the rectum of the frog may serve as a type ot those which feed on decaying debris, and Volvox of those which are holophytic. Radiolarians, with their partner Algae, exhibit the mutual benefits of symbiosis, the plants utilising the carbon dioxide of their transparent bearers, the animals being aerated by the oxygen which the plants give off in sunlight, and moreover nourished by the carbohydrates which they build up. Some of the parasitic forms, especi- ally among the Sporozoa, are fatally injurious to higher animals. Though Protozoa may be seriously infected by Bacteria, by Acineta parasites, by some fungi, like Chytridium, etc., fatal infection is rare, because of the power of intracellular digestion which most Protozoa possess. " The parasite," Metchnikoff says, " makes its onslaught by secreting toxic or solvent substances, and defends itself by paralysing the digestive and expulsive activity of its host ; while the latter exercises a deleterious influence on the aggressor by digest- ing it and turning it out of the body, and defends itself by the secretions with which it surrounds itself." With this struggle should be compared that between phagocytes and Bacteria in most multicellular animals. History. — Of animals so small and delicate as Protozoa, we do not expect to find distinct relics in the much-battered ancient rocks. But there are hints of Foraminifer shells even in the Cambrian ; riiore than hints in the Silurian and Devonian ; and an abundant representation in rocks of the Carboniferous and several subsequent epochs. The shells of calcareous Foraminifera form an important part of chalk deposits. There seem at least to be sufficient relics to warrant Neumayr's generalisation in regard to Foraminifera, that the earliest had shells of irregularly agglutinated particles (Astrorhizida;), that these were succeeded by forms with regularly agglutinated shells, exhibiting types of architecture which were subsequently expressed in lime. PROTOZOA AND DISEASE H5 Remains of siliceous Radiolarian shells are known from Silurian and from Devonian strata onwards. From the later Tertiary deposits of Barbados earth, Ehrenberg described no fewer than two hundred and seventy-eight species. Protozoa and Disease. — The discoveries of recent years have shown that the study of Protozoa is an inquiry of great practical importance. All three main divisions of the Protozoa contain important disease-producing parasitic forms, especially the flagellated Infusorians called Haemoflagellata (Trypanosomes) and the Sporozoon Fig. 67. — Glossina palpalis, tsetse fly. group of Haemosporidia, to w^hich Plasmodium vivax (see Fig. 68) belongs. I . Various species of Amoebae are parasitic in the human food canal, e.g. Entamoeba colt, E. histolytica, lodamoeba butschlii, Endolimax nana, and Entamoeba gingivalis in ill-kept teeth, but the only pathogenic form is Entamoeba histolytica. This Amoeba eats into -the wall of the lower intestine, causing ulceration and, in severe cases, amoebic dysentery as well as abscesses on the liver and elsewhere. A clear cyst may be formed within which the nuclei divide usually into four. The cyst is passed out of the intestine, and should it find its way into the food canal of another human being, the cyst breaks and sets free the contained daughter Amoebae. 10 INFECTIVE MOSQUIToA 6iTes nm. D SCHIZOC-OMY IN MAPS oo riO60UITO B TAhES I- >nGfinE:rocYTes. SPOROGOhY no5Quiro. nOSQUlTObinFECTlVE:. 5toaO20lTES ENTER jQ a. PiQ, 58. — Life-cycle of Plasmodium vivax. — Bas?d]on Sehaudinn. I. Sporozoites; 2-7. Schizogony in human red blood cells ; 7a,8. Formation of gametocytes in red blood cells ; 9. Gametocytes set free in stomach of mosquito ; 10. Production of gametes; 11. Union of i and 9 gametes; 12. Zygote; 13. Zygote burrows through stomach lining of mosquito ; 14. Encysts; 13, 16. Growth and development of sporo- zoites; 17. Rupture of oocyst ; sporozoites set free. SLEEPING SICKNESS 147 2. The Trypanosomidae are flagellate Protozoa, chiefly parasitic in the blood of higher Vertebrates and the ali- mentary canal of Invertebrates. A typical fully formed trypanosome is seen in Fig. 69 (i), but there may be great variation of shape at different stages of the life-history, even rounded non-flagellate stages occurring {Leishmania) . The curved-blade-like cell has a single flagellum rising from a base termed the blepharoplast, and for part of its length joined to the rest of the cell by a thin undulating membrane. Near the blepharoplast is a small nucleus- like body. There is also a prominent central nucleus. Vacuoles and granules may be present in the cytoplasm. A delicate " periplast " covers the outside of the cell. Reproduction is by longitudinal division, beginning at the basal end of the flagellum. Trypanosomes have been found in the blood of many mammals, including mice, voles, rabbits, cavies, squirrels, various bats, moles, shrews, ant-eaters, badgers, marmosets, monkeys, armadillos, as well as the better-known hoofed animals. They are also found in birds, reptiles, amphibians, and a great many fishes. They are spread from one host to another by means of an intermediate host, usually a blood-sucking insect or leech, within which a phase of the life-history is passed. A few occur in plants ! T. gamhiense multiplying in the blood causes African sleeping sickness. It is transmitted by a tsetse fly. T. brucei, also carried by a tsetse fly, and a most virulent trypanosome, causes Nagana in, chiefly, domestic stock. Its " natural " hosts are certain of the bigger African game animals which seem to be unaffected by it. T. equiperdum causes dourine in horses. T. evansi, a trypanosome affecting horses, camels, mules, domestic cattle, and dogs in tropical countries, causes the disease known as " Surra," especially deadly amongst horses. _,In 1907 T. cruzi was discovered by Chagas in South America. It chiefly affects children and adolescents, causing Chagas' disease. It is transmitted by a bug. The commonest trypanosome is that found in the blood of rats — T. lewisi — and transmitted from rat to rat by fleas. When infected blood is sucked in by the flea, the trypanosomes pass from the cavity of the flea's stomach and burrow into the lining cells of 148 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS 3\ UTU Fig. 69. I. Trypanosoma gambiense, showing nucleus (n.) ; blepharoplast (Bl.) ; so-called kineto-nucleus {Bk.) ; undulating membrane (u.m.) ; free end of flagellum (F/.). IL Individual undergoing longitudinal fission. III. and IV. Individual dividing into spores. MALARIA ORGANISM 1 49 the Stomach wall. Here an individual trypanosome may multiply eight or ten fold, till the cell is a mere envelope containing a moving skein of trypanosomes. These then break out into the stomach, and may invade other cells. Some migrate to the rectum and hind gut, whence they are expelled with the excrement. Rats are infected by lick- ing the excrement of the flea while cleaning their fur. Leishmania donovani is a rounded non-flagellate stage of a flagellate parasite, and occurs in the lining cells of blood vessels, causing splenomegaly or Kala azar in tropical and sub-tropical countries. Many attempts have been made to discover the intermediate host of this parasite. Many possible carriers, especially fleas and bugs, have been in- vestigated, but so far without success. L, tropica is the cause of the skin lesions known as oriental sore, a widely distributed disease in warmer countries. Ulcerating wounds develop chiefly in exposed parts of the body, such as hands, feet, and head. The parasites are carried by sand flies. 3. Plasmodium vivax, the organism of benign tertian malaria, has already been dealt with (pp. 125 and 146). As in most Haemosporidia, the schizogony phase of the life- history is intracellular within a red blood cell, while the spo'rogony phase is passed in a carrier insect. The two species P. malaria and P. falciparum cause quartan malaria and malignant tertian malaria respectively. The life-history in each case is very similar to P. vivax. Plas- modium prcecox (Proteosoma) , the parasite of bird malaria, is transmitted by the mosquito, Culex fafigans, and an allied parasite of the pigeon, Hcemoproteus columbce, by a biting fly, Lynchia maura. In Piroplasma {Babesia) bovis the very characteristic first part of the life-history is within the red blood cells of cattle and other animals, the second part — not yet fully understood — within a tick. This minute pear-shaped parasite is the cause of Texas or Red-Water Fever, a formidable cattle disease in certain parts of America and in Australia. Among the other parasitic Sporozoa are various coccidia (Eimeria), which are found in the intestine and related parts of horses, pigs, sheep, and other mammals, of 150 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS grouse, pheasants, and other common birds, as well as a large number of Invertebrates. They are intracellular parasites. Gregarines are common gut parasites of most animals, at first intracellular, later becoming free in the gut cavity. The Spirochaetes form a group of often ex- tremely minute spiral organisms which some regard as Bacteria, others as Protozoa. Some of them are among the most formidable parasites of man. Treponema {Spiro- chceta) pallidum is the cause of syphilis, one of the heaviest taxes on civilisation. Many Spirochastes are Fig. 70. — Colonial Infusorian — Ophrydium sessile. — After vSaville Kent. (Enlarged 100 times.) The individuals are embedded in a jelly-like matrix (/.). M., Mouth ; B., band round oral disc ; C, circumoral cilia. blood parasites, e.^. S. duttoni^ conveyed by a tick, is the cause of African relapsing or tick fever. Another species of Spirochaete, S. recurrenlis, is transmitted in more northern countries in the haemocoele of the body-louse, and causes what is termed " European " relapsing fever. General zoological interest. — The Protozoa illustrate, in free and single life, forms and functions like those of the cells which compose the many-celled animals. Typically, they show great structural or morphological simplicity, but great physiological complexity. Within its single cell the Protozoon discharges all the usual functions, while in a higher animal distinct sets of cells have been specialised for BEGINNINGS ILLUSTRATED BY PROTOZOA 151 various activities, and each cell has usually one function dominant over the others. The Metazoan cells, in acquir- ing an increased power of doing one thing, have lost the Protozoan power of doing many things. The Protozoa remain at the level represented by the reproductive cells of higher forms, and are comparable to reproductive cells which have not formed bodies. In the sexual colonies of Volvox, however, we see the beginning of that difference between reproductive cells and body cells which has become so characteristic of Metazoa The Fio. 71. — A colonial flagellate Infusorian — Proterospongia haeckelii. — After Saville Kent. There are about 40 flagellate individuals, a, Nucleus ; b, contractile vacuole ; c, amoeboid unit in gelatinous matrix ; d, division of an amoeboid unit ; e, flagellate units with collars contracted ; /, hyaline outer membranes ; g, spore-formation. Protozoa are self-recuperative, and in normal conditions they are not so liable to " natural death " as are many-celled animals. Weismann and others maintain that they are physically immortal. They illustrate — (a) the beginnings of reproduction, from m.ere breakage to definite division, either into two, as in fission, or'in limited time and space into many units, as in the formation of spores within a cyst ; (b) the beginnings of fertilisation, from " the flowing together of exhausted cells " and multiple conjugation, to the specialised sexual union of some Infusorians, Heliozoa, Sporozoa, etc. — where two individuals become closely united ; along with this, the 152 PHYLUM PROTOZOA — THE SIMPLEST ANIMALS beginnings of maturation, as shown in the formation of polar nuclei in some Heliozoa, Sporozoa, Flagellata, and Lobosa ; {c) the beginnings of sex, in the difference of size and of constitution sometimes observed between two con- jugating units (e.g. in Coccidium) ; {d) the beginnings of many-celled animals, in the associated groups or colonies which occur in several of the Protozoan classes. These colonies show a gradation in complexity. Raphidiophrys and other Heliozoa form loose colonies, which arise by the close coherence of the products of fission. Among the Radiolarians there are several colonial forms ; in these the individuals are united by their extra-capsular protoplasm, but are all equivalent. In Proterospongia the cells show considerable morphological distinctiveness ; some are flagellate, some amceboid, some encysted and spore- forming. Again, in Volvox, as we noticed above, the cells of the colonies show a distinction into nutritive and repro- ductive units. Lastly, in their antithesis of passivity and activity, con- structive and destructive preponderance, anabolism and katabolism, the Protozoa illustrate the phases of the cell- cycle, and so furnish a key to the variation of higher animals. CHAPTER VIII PHYLUM PORIFERA— SPONGES Class I. Calcarea. Class II. Hexactinellida. Class III. Demospongi^. Sponges seem to have been the first animals to attain marked success in the formation of a " body." For though their details are often complex, their essential structure is simpler than the average of any other class of Metazoa, and some of the simplest forms do not rise high above the level of the gastrula embryo. i\ " body " has been gained, but it shows relatively little division of labour or unified life ; it is a community of cells imperfectly integrated. The cells of the body show an arrangement in two distinct layers (diploblastic). There are no definite organs, and the tissues are, as it were, in the making. Sponges are passive, vegetative animals, and do not seem to have led on to anything higher ; but they are successful in the struggle for existence, and are strong in numbers alike of species and of individuals. General Characters Sponges are diploblastic {two-layered) Metazoa, the middle stratum of cells, the mesogloea, not attaining to the definiteness of a proper mesoderm. There is no coelom or body cavity. The longitudinal axis of the body corresponds to that of the embryo ; in other words, the general symmetry of the gastrula is retained. In these three characters the Sponges agree with the Coelentera, and differ from higher (triplo- blastic and coelomate) Metazoa- '53 154 PHYLUM PORIFERA — SPONGES The body varies greatly in shape, even within the same species. It is traversed by canals, through which currents of water bear food inwards and waste outwards. Numerous minute pores on the surface open into afferent canals, leading into a cavity or cavities lined by flagellate cells, many or all of which have a goblet shape with a delicate collar through which the flagellum rises (" choano- cytes "), To the activity of the fiagella the all-important water currents are due. The internal cavity may be a simple tube, or it may have radially outgrowing chambers, or it may be represented by branched spaces, from which efferent canals lead to the ex- terior. When there is a distinct central cavity there is usually but one large exhalant aperture {psculum), but in other cases there are many exhalant apertures. A delicate outer layer covers the apical oscuium, the inhai- bodv , and IS perhaps inturucd iuto the ant pores in the walls. rr , 7 t^ 7 .1 afferent canals. Beneath the covering layer there is in all but the simplest forms a mass of cells (the mesogloea) which may be very varied in its composition . Thus there are scleroblasts making the skeleton of lime, flint, or spongin ; amoeboid cells or phago- cytes, important in digestion and excretion ; reproductive cells, and other elements. This median mass of cells is traversed by the afferent canals and by the diverticula of the central cavity, or the branches of the original central cavity, lined by flagellate cells. It is difficult to call this cavity or system of cavities the gut or enteron, or to call the layer which lines it the endoderm, or the outer covering layer the ectoderm. In fact, the sponges are very different from other Metazoa, Fig. 72. — Simple sponge {Ascetta primordialis). — After Haeckel. Note the vase-like form, the Fig. 73. — A sponge colony. STRUCTURE OF SPONGES 155 and represent a cul de sac in evolution. There are no nerve- cells — a fatal defect. Budding is very common ^ and in a few cases buds are set adrift. Both herma- phrodite and unisexual forms occur. The sexually-pro- duced embryo is almost always developed within the mesogloea, and leaves the sponge as a ciliated larva. Except the family of Spon- gillids, all are marine. Description of a simple sponge. — A very simple sponge, such as Ascetta, is a hollow vase, moored at one end to rock or seaweed, with a large exhalant aper Fig. 74.— Sponge spicules. Monaxon ; 2, triod ; 3, triaxon ; 4, tet- raxon ; 5, anchor ; 6, polyaxon ; 7, a kind of amphidisc. ture at the opposite pole, and with numerous minute inhalant pores penetrating the walls. In the calca- reous sponges, the pores are minute perforations in single cells (poro- cytes). The walls consist of — (i) a flat covering layer ; (2) a mesogloea con- taining triradiate calcareous spicules, phagocytes, and reproductive ele- ments ; and (3) a layer hning the central cavity, and composed of collared flagellate cells, like some of the monad Infusorians (cf. Fig. 71). More complicated forms. — But a description of a simple sponge like Ascetta conveys little idea of the structure of a complex form such as "■flSS.e'°'"chamber';!"""a the bath-sponge (FAcspongiu). Let us gastruia forming in the cousidcr the Origin of compUcatious mesogloea, etc. / \ r~i i j j {a) Sponges — long regarded as plants — are plant-like in being sedentary and passive. They seem also to feed easily and well. Like plants, Fig. 75. — Section of sponge. — After F. Schulze. is6 PHYLUM PORIFERA — SPONGES CS»^. they form buds, the outcome of surplus nourishment. These buds, Hke the suckers of a rose-bush, often acquire some apparent independence, and the sponge looks like many vases, not like one. Moreover, as they grow these buds may fuse, like the branches of a tree tied closely together. Thus the structure becomes more intricate. {b) In the simple sponge the cavity of the vase is completely lined by the collared flagellate cells {Ascon type). But the inner layer may grow out into radial chambers to which the choano- cytes are restricted (Sycon type), and the walls of these may also be folded into side aisles {Leucon type). The outgrowing of the inner layer into the mesogloea may be con- tinued even further, and the cells may become pavement-like except in the minute flagellate chambers, where alone the char- acteristic choanocytes are retained (see Fig. 76). It may be that the characteristic folding or outgrowth of the inner layer is necessitated by the fact that the com- FiG. 76. — Diagram showing types of canal system. — After Korschelt and Heider. The flagellate regions are dark through- out, the mesogloea is dotted, the arrows show the direction of the currents. All the figures represent cross-sections through the wall. A. Simple Ascon type {Ec, outer layer ; En., inner layer ; Mg., mesogloea). B. Sycon type, with flagellate radial chambers {r.c). C. Leucon type, with flagellate side aisles on the main radial chambers. D. Still more complex type, with small flagellate chambers (f.ch.). STRUCTURE OF SPONGES 157 ponent cells are better nourished and multiply more rapidly than those of the outer layer. (c) By infoldings of the outer layer and a subjacent sheath of mesoglcea — subdermal spaces may be formed ; an outer cortex may be distinctly differentiated from the internal region in which the flagellate chambers occur ; the pores may collect into sieve-like areas, which open into dome-like cavities ; these and many other complications are common. {d) The covering layer usually consists of flat epithelium, but flask-shaped cells have also been observed (Bidder). It may be folded inwards, as we have noticed, and, accord- ing to some, it also lines the inhalant or afferent canals in R.C Fig. 77. — Diagram of sponge structure. R.C., A flagellate chamber into which water passes by an inhalant pore (LP.). O., An osculum, into which the exhalant canals open ; the arrows indicate the directions of the currents. SD.SP., A subdermal cavity or porch, into which the inhalant pores may open. whole or in part. In a few cases, e.g. Oscarella lobularis, it is ciliated, and its cells may also exhibit contractility, as around the osculum of Ascetta clathrus, though the con- tractile elements usually belong to the mesoglcea. The inner layer consists typically of collared flagellate cells, but in the more complex sponges these are replaced, except in the flagellate chambers, by flat epithelial cells, with or without flagella. The mesogloea contains very varied elements, and illus- trates the beginnings of different kinds of tissue. Thus there are migrant amoeboid cells (phagocytes) ; irregular connective tissue cells ; spindle-shaped connective tissue cells, united into fibrous strands ; contractile cells, e.g. those forming a sphincter around the oscula of some forms, 158 PHYLUM PORIFERA — SPONGES such as Pachymatisina ; skeleton-making cells ; pigment- containing cells ; and lastly, the reproductive cells. {e) The skeleton consists of calcareous or siliceous spicules, or of spongin fibres, or of combinations of the two last. A calcareous spicule is formed of calcite, with a slight sheath and core of organic matter ; a siliceous spicule is formed of colloid silica or opal ; the spongin is chemi- cally somewhat like silk. Uniradiate, biradiate, triradiate, quadriradiate, sexradiate, and multiradiate spicules occur, and they are eflfective in keeping the meshes open and in giving the body architectural stability. In every pole scaffolding we see, as it were, huge hexactinellid spicules, spliced together with rope. It is convenient to distinguish the large macroscleres from the small microscleres. Each spicule begins to be formed by one or more " scleroblasts," and may be speculatively regarded as an organised intra- cellular excretion. " During its growth," Professor Sollas says, ** the spicule slowly passes from the interior to the exterior of the sponge, and is finally (in at least some sponges — Geodiiiy Stelletta) cast out as an effete product." The fibres of spongin are formed as the secretions of mesogloea cells, known as spongioblasts. Ordinary functions. — Excepting the fresh-water Spong- illidas, all sponges are marine, occurring from between tide marks to great depths. After embryonic life is past, they live moored to rocks, shells, seaweeds, and the like. Their motor activity is almost completely restricted to the lashing movements of the flagella, the migrations of the phagocytes, and the contraction of muscular mesogloeal cells, especially around the exhalant apertures. In the closure of the inhalant pores, sponges show sensitiveness to injurious influences, but how far this is localised in specialised cells is uncertain. The most important fact in the life of a sponge is that which Robert Grant first observed- — that currents of water pass gently in by the inhalant pores, and more forcibly out by the exhalant aperture or apertures. This may be demonstrated by adding powdered carmine to the water. The instreaming currents of water bear dissolved air and supplies ^i food, such as Infusorians, Diatoms, and particles of organic debris. The outflowing current carries away NUTRITION OF MARINE ANIMALS 1 59 waste. When a sponge is fed with readily recognisable substances, such as carmine or milk, and afterwards sectioned, the grains or globules may be found — (a) in the collared flagellate cells ; {b) in the adjacent phagocytes of the mesogloea ; (c) in the phagocytes surrounding the sub- dermal spaces, if these exist. It is uncertain whether the epithelium of the subdermal spaces or the flagellate lining of the deeper cavities is the more important area of absorp- tion, but it is certain that the phagocytes play an important part in engulfing and transporting particles, in digesting those which are useful, and in getting rid of the useless. In extracts of several sponges, Krukenberg and others have found digestive ferments, probably formed within the phagocytes, but digestion is wholly intracellular. Many sponges contain much pigment ; thus the lipo- chrome pigment zoonerythrin (familiar in lobsters) is common. Some pigments, such as floridine, may help in respiration. The green pigment of the fresh-water sponge is due to the presence of green symbiotic algae {Chlorella), which in their holophytic activity probably supply food- stuflFs to the host. Nutrition of marine animals. — Much discussion has centred round the thesis maintained by Putter, that marine animals find a valuable source of energy in organic com- pounds present in solution in the water. He maintains, for instance, that although a sponge may pass five times its own weight of water through its canals in an hour, yet the particulate, solid food contained in this amount of water is insufficient for the sponge's needs, so that there must be absorption of dissolved food-material. Similar arguments are advanced for higher animals, including fishes, with the corollary assumption that such forms are able to absorb organic substances through their gills or elsewhere. Nearly every fink in Putter's chain of argument has been violently assailed, though it has also found many supporters. It is admitted that sea-water may contain considerable quantities of dissolved organic compounds, but it is uncertain whether any of these are valuable as food-stuffs. The view that there is an insufficiency of solid food is disputed, and unfortunately there has 'been much confusion in the controversy, some l60 PHYLUM PORIFERA — SPONGES workers regarding only whole organisms, and others including colloidal particles, as solid food. There is good evidence that even vertebrates can absorb and utilise dissolved food-materials, if present in relatively high concentration, but it is not clear that such absorption takes place except in the alimentary canal ; in molluscs, it appears to cease if the mouth is stopped up. Owing to the difficulty of excluding bacteria, the experimental evidence is not truly decisive ; and it is very doubtful whether dissolved substances play an important part in nutrition. On a rather different level are the experiments of Peters, who got various Infusoria to thrive on nutrient solutions, apparently free from soUd particles or bacteria. Nitrogen and carbon were present in the solution only in the re- latively simple compound ammonium glycero-phosphate ; rnost higher animals undoubtedly require more complex nitrogen- and carbon-containing compounds (amino- acids, at least). Further work on these lines would be very valuable. Reproduction. — If a sponge be cut into pieces, these may regenerate the whole — a fact which illustrates the relatively undifferentiated state of the sponge body. It is possible that fission may sometimes occur naturally. Ordinary budding is a mode of continuous growth, but when small buds are set adrift, e.g. in Donatia and Tethya, there is a form of asexual reproduction. In the fresh-water Spongilhdae there is a pecuUar mode of reproduc- tion by statoblasts or gemmules. A number of mesogloeal cells occur in a clump, some forming an internal mass, others a complex protective capsule, with capstan-like spicules, known as amphidiscs. According to W. Marshall, the life-histor\^ is as follows : In autumn the sponge suffers from the cold and the scarcity of food, and dies away. But throughout the moribund parent gemmules are formed. These survive the winter, and in April or May they float away from the dead parent, and develop into new sponges. Some become short-lived males, others more stable females. The ova produced by the latter, and fertilised by spermatozoa from the former, develop into a summer generation of sponges, which, in turn, die away in autumn, and give rise to gemmules. The Ufe-history thus illustrates what is called alternation of genera- tions. Interpreted from a utihtarian point of view, the formation of gemmules is a Hfe-saving expedient. As Professor SoUas says, " the gemmules serve primarily a protective purpose, ensuring the persistence of the race, while as a secondary function they serve for dispersal." DEVEI.OPMENT OF SPONGES l6l All sponges produce sex cells, which seem to arise from amceboid mesoglcea cells retaining an embryonic character. In the case of the ovum, the amoeboid cell increases in size, and passes into a resting stage ; in the case of the male elements, the amoeboid cell divides into a spherical cluster of numerous minute spermatozoa. The similar origin of the ova and spermatozoa is of interest. Most sponges are unisexual, but many are hermaphrodite. In the latter case, however, either the produc- tion of ova or the production of spermatozoa usually preponder- ates, probably in dependence upon nutritive conditions. Development. — It is not sur- prising to find that there is great variety of development in the lowest class of Metazoa ; it seems almost as if numerous experi- ments had been made, none attended with progressive success. The minute ovum, without any protective membrane, usually lies near one of the canals, and is fertilised by a spermatozoon borne to it by the water. It exhibits a certain power of migration, as in some Hydroids. Previous to fertilisation, the usual extrusion of polar bodies has been observed in a few cases, Fig. 78. — Development of Sycandra raphanus. — After F. E. Schulze. 1. Ovum. 2. Section of i6-cell stage. 3. Blastula with 8 granular cells {gr.c.) at lower pole. 4. Free-swimming amphiblastula, with upper hemisphere of flagellate cells (f.c.), and lower hemisphere of granular cells {gr.c). 5. Gastrula stage settled down. Ec, Outer layer ; En., inner layer ; bl., closing blastopore ; am.p., mooring, amoeboid processes. II ■gf.C. am.p 1 62 PHYLUM PORIFERA — SPONGES and is doubtless general. Segmentation is total and usually equal, and results in a spherical or oval embryo more or less flagellate. This leaves the parent sponge, swims about for a time, then settles down, and undergoes a larval meta- morphosis often diflicult to understand. It is peculiarly difficult to bring the history of the germinal layers in sponges into line with that in other Metazoa. {a) In the small calcareous sponge Sycandra raphanus (Fig. 78), of the Mediterranean, whose development is very similar to that of the common British Sycon ciliatum, the segmentation results in a hollow ball of cells — the blastula. A few cells at the lower pole remain large, and are filled with nutritive granules ; the other cells divide rapidly and become small, clear, columnar, and flagellate. The large granular cells be- come invaginated, at first, \mtil the embryo is free of the parent, when the rounded form is restored in the amphi- blastula (Fig. 78, 4). This swims for a time actively, but the flagellate cells of the upper hemisphere are invaginated into or overgrown by the large granular cells, and thus what is generally called the gastrula stage results. This soon settles down, on rock or seaweed, with the blastopore or gastrula mouth down- FiG. 79.— Diagrammatic re- wards, and is moored by amoeboid pro- presentation of develop- messes from the granular cells, which ment of Oscarella lobularis. likewise obliterate the blastopore. The After Heider. granular cells lose their granules, for the Bl., Free-swimming blastula with larva is not yet feeding ; the flagellate flagella ; G., gastrula settled ^ells begin to acquire the characteristic NexTTgure shows folding of inner collar ; a mesoglcea with spicules begins layer (En.) ; Ec, outer layer. to be formed between the inner and outer Lowest figure shows radial cham- layer, probably by migrants from the bers {R.C.) ; Mesoglcea (Mg.) ; i„4.*p_ Pores ooen through the walls inhalant pore (P.) ; exhalant ^atier. rores open xnrougn ine waus, osculum (0.). water is drawn m by the action of the flagella, and an exhalant aperture is ruptured at the upper pole. The young sponge i^ now in an A scon stage, from which, by the outgrowth of the inner flagellate layer into radial chambers, it passes into the permanent Sycon form, heightens, and becomes differentiated in detail. (b) In Oscarella (Halisarca) lobularis (Fig. 79), a sponge without any skeleton belonging to the Demospongise, the ovum seements equally into a blastula, which is flagellate all over. During this free-swimming DEVELOPMENT OF SPONGES 163 stage the cells at one pole lose their flagella and become granular, and an amphiblastula results. This invaginates to form a hemispherical gastrula, which settles mouth downwards. Pores, an osculum, and the mesogloea are formed as before, and the inner layer becomes folded into flagellate chambers. The main features of sponge embryology are thus summarised by Minchin : — " I. The larva is composed of three classes of cell-elements : (i) Columnar flagellated cells, forming the outer covering or localised at the anterior pole ; (2) rounded, more or less amoeboid elements, rarely flagellated, forming the inner mass or aggregated at the posterior pole ; and (3) the archaeocytes, usually scattered in the inner mass, and often represented by undifferentiated blastomeres. . . . " II. The larva fixes and undergoes a metamorphosis whereby the Fig. 80. — Diagram of early iixed stage of sponge. D., Dermal layer of cells ; G., gastric layer of cells ;• BL., cavity of blastula disappearing ; A., archen- teron ; P., attaching processes of the outer layer cells. This early stage is somewhat like a thimble, fastened mouth (M.) downwards to the substratum. flagellated cells become placed in the interior, while the cells of the inner mass come to surround them completely. '■ III. (i) The flagellated cells of the larva become the choanocytes of the adult (gastral layer), acquiring a collar ; . . . (2) the inner mass gives rise to the dermal layer in its entirety : . . . (3) the archaeocytes become the wandering cells of the adult, from which the reproductive cells arise." It is interesting to note that the primitive germ-cells are early set apart. Classification. — Class I. — Calcarea. With skeleton of calcareous spicules : — Grade I. — Homoccela. — Continuous internal layer of collared flagellate cells, e.g. Ascetta, Leucosolenia. Grade II. — Heterocoela. — Collared flagellate cells restricted to radial tubes or chambers, e.g. Sycon (Grantia). 164 PHYLUM PORIFERA — SPONGES Class II. — Hexactinellida, or Triaxonida, with sexradiate siliceous spicules (triaxons). The members live chiefly in deep water, e.g. Venus Flower-Basket (Euplectella) and the Glass-Rope Sponge (Hyalonema). Class III. — Demospongi^. Skeleton of siliceous spicules, but never triaxons, or of spongin fibres, or of spongin fibres and siliceous spicules, or absent. Grade I. — Tetraxonida, typically with tetraxon spicules, e.g. Pachymatisma, Tetilla. Grade II. — Monaxonida, with monaxon spicules, sometimes with spongin in addition, e.g. Mermaid's Gloves {Chalina oculata), Crumb-of-Bread Sponge (Halichondria or Amorphina panicea), Fresh-Water Sponge (Spongilla). Grade III. — Ceratosa, " horny " sponges with or without spicules, e.g. the Bath-Sponge {Euspongia). Grade IV. — Myxospongida, without any skeleton, e.g. Halisarca and Oscarella. A very remarkable form called Merita seems to have both a siliceous and a calcareous skeleton. History. — Sponges, as one would expect, date back almost to the beginning of the geological record. Thus the siliceous Protospongia occurs in Cambrian rocks, and in the next series — the Silurian — the main groups are already represented. From that time till now they have continued to abound and vary. The division between calcareous and siliceous sponges goes deep down to the very roots of the phylum, and the siliceous branch must have divided very early into Triaxonida and Tetraxonida. Ecology. — Sponges are living thickets in which many small animals play hide-and-seek. Many of the associa- tions are harmless, but some burrowing worms do the sponges much damage. The spicules and a frequently strong taste or odour doubtless save sponges from being more molested than they are ; the numerous phagocytes wage successful war with intruding micro-organisms. Some sponges, such as Cliona on oyster-shells, are borers, and others smother forms of life as passive as themselves. Several crabs, such as Dromia^ are masked by growths of sponge on their shells, and the free transport is doubtless advantageous to the sponge till the crab casts its shell. A compact orange-coloured sponge {Suberites domuncula) of peculiar odour often grows round a whelk-shell tenanted by a hermit-crab, and gradually dissolves the shell-sub- stance. Within several sponges minute Algae live, like the " yellow cells " of Radiolarians, in mutual partnership or symbiosis. One of the cuttlefishes, Rossia glaucopis, puts its eggs carefully into pockets in the substance of a siliceous POSITION OF SPONGES 165 sponge. Finally, sponges deserve mention as factors in human civilisation. General zoological position. — Sponges form the first successful class of Metazoa. They illustrate the beginnings of a *' body," and the beginnings of tissues. Along with B Fig. 81. — A. Young Dicyema.— After Whitman. B. Female Orthonectid (Rhopalura giar- dii). — After Julin. e., Ectoderm ; en., inner endoderm cell with nucleus [n.) ; and embryo [em.). Note the segmentation and the fibrillation supposed to be muscular. Fig. 82. — Salinella. — After Frenzel. 1. Longitudinal section — a., anterior ; p., pos- terior. 2. Transverse section. the Coelentera, they differ markedly from the triploblastic, coelomate Metazoa, which do not retain the radial sym- metry of the gastrula. In their germinal layers and in their internal cavity they differ so much from Coelentera and all other Metazoa, that they must be regarded as on a by-road of evolution. This has been emphasised by 1 66 PHYLUM PORIFERA — SPONGES Sir E. Ray Lankester in the term '' Parazoa." He also speaks of them as a sterile stock. Their origin is wrapped in obscurity ; it may be that they are the non-progressive descendants of primitive gastrula-like ancestors with a sluggish constitution. The presence of choanocytes suggests a relationship with certain of the flagellate Protozoa (Choanoflagellata), and Protero- spongia (Fig. 71) may possibly be regarded as a connecting link. Incertte Sedis. Mesozoa The title Mesozoa was applied by Van Beneden to some simple organisms which appear to occupy a very humble position in the Metazoan series. He regarded them as intermediate between Protozoa and Metazoa ; but others have remarked on their resemblance to Platyhelminthes, and especially to the sporocysts of certain Flukes. They may perhaps be regarded as precociously reproductive sporocysts. It will be enough here merely to notice four types : — 1. Dicyemidse (type Dicyema) occur as parasites in Cephalopods ; the body consists of a ciUated outer layer, enclosing a single multi- nucleate inner cell, within which egg-like germs develop, apparently without fertilisation, into dimorphic embryos (see Fig. 81, A). 2. Orthonectidas (type Rhopalura) occur as parasites in Turbellarians, Brittle-stars, and Nemerteans ; the body is slightly ringed, and con- sists of a ciliated outer layer, a subjacent sheath of contractile fibres, and an internal mass of cells, among which ova and spermatozoa appear. The sexes are separate and dimorphic (see Fig. 81, B). 3. Professor F. E. vSchulze discovered a small marine organism — Trichoplax adhcsrens — in the form of a thin, three-laiyered, externally ciliated plate ; and Monticelli records a similar form under the title Treptoplax adhcsrens. But Trichoplax is now said to be the planula of the Hydromedusan Eleutheria. 4. Professor J. Frenzel discovered in brine solutions a minute Turbellarian-like organism — Salinella salve — whose body consists of one layer of cells (Fig. 82). There is an anterior mouth, a ciliated food canal, and a posterior anus. The ventral surface is finely ciliated, the other cells bear short bristles. The animal reproduces by trans- verse fission, but conjugation and encystation also occur. It must be confessed that some corroborative evidence in regard to this peculiarly simple animal is much to be desired. CHAPTER IX PHYLUM CCELENTERA Class I. Hydrozoa. Hydroids and Medusoids. Class 2. ScYPHOMEDUSvE or ACRASPEDA. Jelly-fishes. Class 3. Anthozoa or ACTINOZOA. Sea-anemones, Madrepore-corals, Alcyonarians, etc. Class 4. Ctenophora. The Coelentera — including zoophytes, swimming-bells, jelly-fish, sea-anemones, Alcyonarians, corals, and the like — form a very large series of Acoelomate Metazoa, i.e. multicellular animals without a body cavity. Their simplest forms are not much above the level of the simplest sponges, but the series has been more progressive. Thus many illustrate the beginnings of definite organs. In their variety they seem almost to exhaust the possibilities of radial symmetry, and some types {e.g. Ctenophora) may be regarded as pioneers of the yet more progressive bilateral " worms." Many are very vegetative, deserving the old name of zoophytes (which should rather be read back- wards — Phytozoa), and in their budded colonies afltord interesting illustrations of co-operation and division of labour. With the exception of three or four fresh-water forms like Hydra, all are marine. General Characters The Coelentera are almost always radially symmetrical animals in which the primary long axis of the gastriila becomes the long axis of the adult. There is no body cavity, or coelom, distinct from the digestive cavity {enteron) and its outgrowths. In the lower members of the phylum, the 167 1 68 PHYLUM CCELENTERA primary opening of this cavity becomes the mouth of the adult ^ but in the more specialised types there is an (ectodermic) oral invagination, which forms a gullet-tube or stomodteum. Between the ectoderm and endoderm of the body wall there is a supporting layer, or mesogloea, often of jelly-like con- sistency. In Ctenophora, however, a more definite mesoderm is established at an early stage in development. In the simplest cases the mesogloea is a secretion quite devoid of cells, but secondary cells may migrate into it from the endoderm. Stinging cells of varying complexity are almost always present, but in most of the Ctenophora their place is taken by adhesive cells. The Coelentera exhibit two types of structure — polypoid and medusoid — which recur in modified forms throughout the group, and may be both present in the course of one life- history, when they illustrate the phenomenon of alternation of generations , or metagenesis . The more primitive type is the sessile tubular polyp, which, at its simplest, may be com- pared to a gastrula fixed by one end, and furnished with a crown of tentacles round the central aperture of the other pole. The other derived form, which has become specialised in various directions, is the active medusoid or jelly-fish type. In several divisions the formation of a calcareous " skeleton " by the polypoid type results in the production of " corals.'* Multiplication by budding is common, and often results in the formation of colonies, some of which show considerable divi- sion of labour. The preservation of the primary axis, the absence of true mesoderm and of a ccelom, are often said to distinguish Coelentera and Sponges from the other Metazoa {Coelomata), but the results of recent researches on the nature of the meso- derm seem to rob this distinction of part of its precision. General Survey The CcElentera or " Stinging animals " include a large number of familiar and beautiful forms. The graceful zoophytes which fringe shells and stones, and the tiny transparent bells which float in the pools ; the sea-anemones which cluster in the nooks of the rocks, and the active jelly- fish which swim on the waves, are but different expressions CCELENTERATE STRUCTURE 169 of the antithesis between sedentary polypoid and active medusoid types which is characteristic of the phylum. The dehcate iridescent globes, which represent the class Ctenophora, illustrate the climax of activity, but two or three give indications of sedentariness. In our preliminary survey of the series, we may begin with the little fresh- water Hydra (Fig. 87), which is often Fig. 83. — Diagram of Coelenterate structure, endoderm darker throughout 1. To left, shows longitudinal section of Hydra ; to right, of sea-anemone, g.. Gut ; gl., incipient gullet. 2. To left, shows cross-section of Hydra ; to right, of sea- anemone, in the region of the gullet ; mesenteries not shown. 3. To left, shows vertical section of Craspedote Medusoid (with velum) ; to right, of Acraspedote Medusa (with- out velum), g.. Gut ; g/., gullet. Note anatomical correspondence of the polypoid and medu- soid forms. to be found attached to the stems- and leaves of water- plants. The structure here is extremely simple, but the simplicity is probably due to degeneration. In favourable conditions the polyp may give off daughter buds, which remain for a time attached to the parent, and then separate as independent polyps. The bud itself, before leaving the parent, may also bud, so that three generations are present. If we picture this process of gemmation, but with 170 PHYLUM CCELENTERA imperfect separation of the units, continued indefinitely, we can understand the formation of hydroid colonies, such as the zoophytes. In such cases the colony is usually sup- ported by an organic sheath (perisarc) of varying complexity. But the members of such a colony do not usually remain similar and equivalent. In Hydr actinia, for example, which often grows on a Gastropod shell tenanted by a hermit- crab, the colony consists of polyps of varied structure and function. Some of the polyps are nutritive " persons," like Hydra in appearance ; some are reproductive " per- FiG. 84. — Colony of Hydractinia on back of a Buccinum shell tenanted by a hermit-crab. sons," with rudimentary tentacles, with or without a mouth ; others are long, slender, mobile, sensitive, often abundantly furnished with stinging cells ; while the little protecting spines at the base of the colony may perhaps be abortive " persons." All these polyps are united by con- necting canals at the base. Thus Hydractinia exhibits polymorphism among the members of the colony, and a tendency towards more or less division of labour is common in the Coelentera. In most hydroid colonies the division of labour only amounts to dimorphism ; there are reproductive " persons," different from the ordinary polyps. These are in many COLONIES OF CCELENTERA 171 cases sessile and mouthless, or they may after a time become detached and float away as deHcate, pulsating swimming-bells. These swimming-bells are male and female, they give rise to male and female elements, and so to embryos, which, after a time, settle down and form new zoophyte colonies. This is an instance of alternation of generations. Again, just as the predominance of passivity is exhibited in Hydractinia and some zoophytes, where the active swimming - bell stage is left out of the life-history, so the pre- dominance of activity is exhibited in the per- manent medusoids, e.g. Geryonia^ where the sedentary hydroid stage is omitted, and the em- bryo becomes at once medusoid. Finally, the medusoids themselves may become- colonial, and we have active floating colonies, like those of the Portuguese man-of-war, which show, on a diflferent plane, as much polymorphism as Hydractinia. The same general con- clusions apply to the jelly-fish and sea-anemones. The jelly-fish present a strong resemblance to the medusoids, but are distinguished from them by their usually greater size, as well as by greater complexity and several anatomical differences. It is in accordance with this increased complexity that the alter- nation of active and passive forms, though as real, is less obvious. But even here we find one type {Pelagia) always locomotor, another (Aurelia) whose early life is sedentary, and others (Lucernarians) which in their adult life are Fig. 85. — Diagram of a typical Hydrozoon polyp. — After Allman. EC, Ectoderm ; EN., endoderm ; C, the cavity of the gut (coelenteron) ; G., a re- productive bud ; T., a tentacle ; H., hypo- stome or oral cone ; M., mouth. 1-72 PHYLUM CCELENTERA predominantly passive, and attach themselves by a stalk. The sea-anemones and their numerous allies may be regarded as bearing to the jelly-fish a relation somewhat similar to that which the hydroid polyps bear to the swimming-bells (Fig. 83). They are, however, much more complicated in structure than the hydroids. Solitary forms are much commoner than in the hydroids, but the colonial type is nevertheless very frequent. The colonies may be supported by an organic framework only, but very commonly there is a tendency to accumulate lime in the tissues, which results in the formation of " corals." It should be noted, however, that various quite distinct polypoid types may form " corals." Thus, while the most important reef-building corals are included in the Anthozoa, the Millepore-corals are hydroids. Finally, as the corals are predominantly passive, so there is a climax of activity in the Ctenophores, which move by cilia united into combs, and often shine with that " phos- phorescence " which is an expression of the intensity of life in many active animals. As to diet, many of the larger forms, e.g. sea-anemones and jelly-fish, are able to engulf booty of considerable size ; the active Ctenophores are carnivorous, attaching them- selves by adhesive cells to one another, or to other small animals ; most Ccelentera feed on minute organisms, in seizing and killing which the tentacles and stinging cells are actively used. Stinging cells or cnidoblasts are so characteristic of Coelentera that they deserve particular notice. They occur in all Coelentera except the Ctenophores, and even there they have been detected in Euchlora rubra. They also occur in some Turbellarian worms, and in the papillae of ^olid nudibranchs amongst molluscs ; but it has been shown that these animals obtain their nematocysts from the Coelentera on which they feed. Each cnidoblast contains a capsule or nemato- cyst, which encloses a coiled lasso lying in an irritant gelatinous substance. The nematocyst fills most of the cell. At the distal end of the cell there may be a trigger-like cnidocil or a fringe of bristles. At the proximal end there may be fixing processes. In some Anthozoa the coiled lasso is simply ruptured out, but in most cases it is evagin- ated. The basal part of the lasso is often stronger than the rest, and may bear barbs or stilets ; spirally arranged roughnesses and bristles are also frequent on the thread itself. The explosion of the cnidoblast is believed to be due to aji entrance of water, which causes STINGING CELLS 173 the gelatinous substance to swell up. According to others, the cnidoblast contracts as a whole. The action of the threads is mechanical and chemical. They fix, e.g. by the stilets, into the victim, and the secretion poisons the wound, paralysing or killing small animals, and sometimes acting as a solvent. Many seem to be pre- hensile threads rather than weapons. The nervous system of the Coelenterates is of a primitive type : a network of nerve-fibres runs diffusely and almost uniformly through the body ; thus, as Romanes showed, it is possible to cut a jelly-fish into a fantastic pattern, a long ribbon for instance, without preventing the conduc- FiG. 86. — Diagram of stinging-cells or cnidoblasts, the one to the right undischarged. I, Nucleus ; 2, cytoplasm ; 3, lasso or nematocyst, with barb-like processes at its base ; 4, the fluid-containing cavity of the cell in which the undischarged nematocyst lies coiled up ; 5, the trigger or cnidocil. tion of nervous impulses from one end to the other ; whereas in Vertebrates an injury to the spinal cord at once cuts off the lower part of the body from all nervous com- munication with the brain. The Coelenterates have no central nervous system, but only a nerve-net ; but in the Anthozoa and Scyphomedusae there may be regions over which the nerve-net does not extend, there may be differ- entiated organs of special sensitiveness, and there are usually certain nerve-tracts in which conduction of the impulse takes place rapidly and in a determined direction — hinting at the definite localised nerves of higher phyla. 174 PHYLUM CCELENTERA Many points in the behaviour of the Ccelenterates may be deduced from the structure of the nervous system and the absence of any co-ordinating centre ; they are " reflex repubhcs," in which any excited portion may acquire temporary dominance over the rest. Nerve-networks of a similar nature are found in the walls of the viscera of Vertebrates, e.g. Auerbach's plexus. Types of Ccelentera First Type — Hydra, a simple representative of the Class Hydrozoa General life. — The genus Hydra — cosmopolitan, like many other small fresh-water animals — is represented by several species, e.g. the green Hydra viridis, the brownish H. oligactis or fusca, and the orange H. vulgaris or grisea, widely distributed in fresh water. They are among the simplest of Ccelentera, for the body is but a two-layered tube, with a crown of (6-10) hollow tentacles around the mouth, and with no organs except those concerned in re- production. The body is usually fixed by its base to some aquatic plant, often to the lower surface of a duckweed. It may measure J- J inch in length, but it is as thin as a needle, and contracts into a minute knob. The animal sways its body and tentacles in the water, and it can also loosen its base, lift itself up by its tentacles, stand on its head, or creep by looping movements. According to some observers, its movements are helped by fine pointed pseudopodia protruded from the ectoderm cells of the tentacles and base, and by threads ejected from large cylindrical stinging cells. Usually, however, the Hydra prefers a quiet life. It feeds on small animals, which are paralysed or killed by stinging cells on the Fig. 87. — Hydra hang- ing from water-weed. — After Greene. ov., Ovary ; t., testes. STRUCTURE OF HYDRA I75 tentacles, and are swept into the tubular cavity of the body by the action of flagella on the internal cells. Sometimes animals as large as water-fleas {e.g. Daphnid) are caught, and the Hydra may sometimes be seen struggling fiercely with a small Annelid worm (Tubifex). Infusorians (EtiploteSy etc.) are often seen wandering to and fro on the surface of the Hydra, but these wonted visitors do not provoke the stinging cells to action. So simple is Hydra, that a cut-oflF fragment may grow into an entire animal. Thus the Hydra may be multiplied by being cut in pieces. The two conditions of a fragment regenerating a whole are — (i) that the fragment be not too small, and (2) that it be a fair sample of the various kinds of cells in the body. Thus neither a little corner off the base nor the tip of a tentacle will grow into a new Hydra. If the animal be turned inside out (a delicate operation), the status quo is soon restored. The Abbe Trembley, who first made this experiment, thought that the out-turned endoderm assumed the characters of the ectoderm, and that the inturned ectoderm assumed the characters of endoderm. But this is not the case. Either the animal rapidly rights itself by turning outside in, or, if this be prevented, the inturned ectoderm disappears internally, and by growing over the out-turned endoderm, from the lips downwards, restores the normal state. In favourable nutritive conditions, the Hydra forms buds, and on these a second generation of buds may be developed. A check to nutrition or some other influence causes the buds to be set adrift. Sometimes a Hydra divides across the middle, and each half grows into a complete polyp in a few days. Besides these asexual modes of multiplication, the usual sexual reproduction occurs. General structure. — The tubular body consists of two layers of cells, i.e. the animal is diploblastic. The cavity is the gut, and it is continued into the hollow tentacles. These, when fully extended, may be much longer than the body. The mouth is slightly raised on a disc or hypostome. Of the two layers of cells, the outer or ectoderm is trans- parent, the inner or endoderm usually contains abundant pigment. On the tentacles especially, even with low power, one can see numerous clumps of clear stinging cells. The 176 PHYLUM CCELENTERA male organs appear as ectodermic protuberances a short distance below the bases of the tentacles ; the ovary, with a single ovum, is a larger bulging farther down. Both male and female organs may occur on the same animal, either at one time or at different times, but often they occur on different individuals. Abundant food favours the develop- ment of female forms ; when food is scarce males are more abundant. The buds have the same structure as the parent body ; in origin they appear to be mainly due to multiplica- tion of interstitial cells. Minute structure. — The outer layer or ectoderm includes the following different kinds of cells : — (i) Large covering or epithelial cells, within or between some of which lie the stinging cells. The epithelial cells are somewhat conical, broader externally than internally, and in the interspaces lie interstitial cells. By certain methods, a thin shred can be peeled off the external surface of the ectoderm cells. This is a cuticle, i.e. a pellicle no longer living, produced by the underlying cells. (la) Many of these large cells have contractile basal processes, or roots, running parallel to the long axis of the body, and lying on a middle lamina which separates ectoderm from endoderm (Fig. 88, E). The cells themselves are contractile, but there are these special con- tractile roots. Like the muscle cells of higher animals, they contract under certain stimuli, and are often called " neuro-muscular." But the presence of special nerve cells shows that even in Hydra there is a differentiation of the two functions of contractility and irritability. (2) Stinging cells or cnidoblasts occur abundantly on the upper parts of the body, especially on the tentacles. Under stimulus, whether directly from the outside or from a nerve cell, the cnidoblast explodes and the nematocyst is thrown out. With the help of the barbs they penetrate through even a chitinous membrane, and the secreted fluid has a solvent action. The victim is held fast and drawn closer. Besides the ordinary stinging cells, there are others of small size which coil into a spiral after explosion. (3) There is to the inner aspect of the covering cells a network of ganglion cells and nerve processes. More superficially there are minute sensory cells, some of them connected by fine fibres with the ganglion cells. (4) Small interstitial or indifferent units fill up chinks in the ecto- derm, and seem to grow into reproductive, stinging, and other cells. (5) Granular glandular cells on the basal disc or " foot " probably secrete a glutinous substance. They are also said to put out pseudo- podia, and so move the animal slowly. The endoderm is less varied. Its cells are pigmented, often vacuolated, and most of them are either flagellate or amoeboid. The pigment bodies in H. viridis are like the chlorophyll corpuscles of plants ; it seems almost certain that they are unicellular Alga;. When a green Hydra liberates an egg while kept in the dark, that egg gives STRUCTURE OF HYDRA 177 rise to a white Hydra, which is supposed to imply that the partner Algse do not migrate into the egg when there is no hght. In the other species of Hydra, the pigment is quite different from chlorophyll. The active lashing of the flagella causes currents which waft food in and waste out. If some small animal, stung by the tentacles, is thus wafted in, it may be directly engulfed by the amoeboid processes of some of the cells, and it has been noticed that the same cell may be at — ^■-..^■ End /TI JtC -«^^^^-;; B Fig. 88. — Minute structure of Hydra.— Ki\.QX T. J. Parker and Jickeli. A. Ect., Ectoderm ; mg., mesogloeal plate ; %i.c., stinging cell ; End., endo- derm with flagella and amoeboid processes. B. M.C., Nerve cell, and st.c, stinging cell. C. Stinging cell with ejected thread ; n., nucleus. D. Mesogloeal plate (mg.) with contractile roots resting on it. E. m.c. Muscular cell with contractile roots, c.r. one time flagellate and at another time amoeboid (cf. the cell-cycle, Fig. 59). After this direct absorption the food is digested within the cells, and while some of the dark granules seen in those cells may be decomposed pigment bodies, others seem to be particles of indigestible debris. Thus Hydra illustrates what is called intracellular digestion, such as occurs in Sponges, some other Coelentera, and some simple " worms." But experiments show that some of the protein of the food may be digested in the gut cavity, and subsequently absorbed. Thus it seems that both intracellular and extracellular digestion occur. 12 178 PHYLUM COELENTERA There is no fundamental physiological difference between the two processes ; a food-vacuole is as certainly a " dead space " as a cavity lined by cells is. In some flagellate Protozoa digestion takes place in a single vacuole of very large size ; these may be regarded as physiologically transitional types. Some of the endoderm cells have muscular roots like those of the ectoderm. They lie on the inner side of the middle lamina, in a trans- verse or circular direction. A few cells near the mouth and base are described as glandular, and the presence of a few stinging cells has been recorded, though some suggest that the last are discharged ecto- dermic nematocysts which have been swallowed. The middle lamina, representing the mesogloea, is a thin homo- geneous plate, bearing on its outer and inner surfaces the muscular roots of ectodermic and endodermic cells (Fig. 88, D). It is historically interesting to notice the important step which was made when, in 1849, Huxley definitely compared the outer and inner layers of the Coelentera with the epiblast and hypoblast which embry- ologists were beginning to demonstrate in the development of higher animals. Not long afterwards, Allman applied to the two layers of hydroids the terms ectoderm and endoderm ; and these are now used embryologically. The division of labour among the cells of Hydra is not very strict, but already the essential characteristics of ectoderm and endoderm are evident. We use ectoderm and epiblast, endoderm and hypoblast, as synonymous. Outer Layer. Middle Layer. Inner Layer. In Hydra the ectoderm forms — Covering cells, stinging cells, nerve cells, muscle cells, etc. None in Hydra, apart from the middle lamella. In Hydra the endoderm forms — Digestive cells lining the food canal, and also muscle cells, etc. The embryonic epiblast of higher animals grows into epidermis, nervous system, and essential parts of sense organs. The mesoblast of higher animals becomes muscu- lar, connective, and skele- tal tissue. The embryonic hypo- blast of higher animals always lines the digestive part of the food canal. The reproductive organs. — [a) From nests of repeatedly dividing interstitial cells, several (1-20) simple male organs or testes are formed. Each consists merely of a clump of male elements or spermatozoa, bounded by the distended ectoderm. Through this the spermatozoa are extruded at intervals, and one may fertilise the ovum of the Hydra. In other words, self-fertilisation, which is very rare among animals, may occur. The spermatozoon is a motile cell, with a minute cylin- drical " head " consisting of nucleus, a more minute middle-piece, and a long thread-like vibratile tail (Fig. 89, i). (ft) Usually there is but one female organ or ovary, but in H. fusca as many as eight have sometimes been observed. The ovary arises, like DEVELOPMENT OF HYDRA 179 the testes, from a nest of interstitial cells, in the centre of which, distinct from the start, the single ovum lies. In rare cases in H. viridis, H. fusca, and H. grisea there are two ova ; in H. dicecia there may be several. Development. — The ovum of Hydra is the successful central cell in the ovary. It is at first amoeboid, and becomes more and more rich at the expense of its neighbours. Their remains (perhaps nuclei) accumulate within the ovum as " yolk spherules " or " pseudo-cells." Some yolk-granules, formed within the ovum, may coalesce in " pseudo- cells " of another type. With increase of size the ovum changes its \. ' '.«•' '•;'.••• '-'W' -5 o >rf3 » » • • » • • t , • • • . j . x» ••• •-/ ' -x •/ / . . • 1^>k "■+ end Fig. 89. — Development of Hydra. — After Brauer. 1. sp.. Spermatozoa. 2. Amoeboid ovum ; g.v., germinal vesicle or nucleus ; y.s., yolk spherules. 3. Ovum with lobed envelope (sh.) around it. 4. Ovum protruding ; w.. the nucleus; ec/., the ruptured ectoderm; end., the endoderm. 5. Section of blastula or blastosphere — Ect., ectoderm ; End., endoderm — being formed. 6. Section of young Hydra. Ect., Ectoderm ; End., endoderm ; g.c, gut cavity ; sh., ruptured envelopes. form from amoeboid to cake-like, and from that to spherical. Around the spherical ovum a gelatinous sheath is formed. When the limit of growth is reached, the nucleus or germinal vesicle divides twice, and two polar bodies are extruded at the distal pole. There are twelve chromosomes to begin with, and by the reduction division in forming the first polar body, the number is reduced to six. Thereafter the ectoderm of the parent Hydra yields to the increasing strain put upon it, and ruptures, allowing the ovum to protrude. By a broad base it still remains, however, attached to the parent, and in this state it is fertilised, the spermatozoon entering by the distal pole (Fig. 89, 4). l8o PHYLUM CCELENTERA The segmentation which follows is total and equal, and results in the formation of a blastula (Fig. 89, 5). By inwandering, or by division of the cells of the blastula, an internal endoderm is formed, and this formation takes place on all sides. In a word, it is multipolar. The segmentation cavity of the blastula is thus filled up, and the two layers become differentiated from one another. The outer or ectodermic layer forms — (a) an external " chitinoid " shell of several layers ; (b) an internal membrane, homogeneous, thin, and elastic ; and (c) the future ectoderm of the adult. In Hydra fusca the egg is separated from the parent before the shell is formed, and is fastened by its gelatinous sheath to aquatic plants ; in H. viridis and H. grisea the egg falls off after the outer shell has been formed. In all species the separation from the parent appears to be followed by a period of quiescence lasting from one to two months. It is probable that this resting-stage is carried by wind and birds from one water basin to another. Within the shell differentiation at length recommences, but it pro- ceeds slowly. Interstitial cells arise in the ectoderm ; a middle lamella is formed ; a gastric cavity begins to appear in the midst of the endoderm. Thereafter the shell bursts, and development proceeds more rapidly. The embryo elongates, acquires a mouth by rupture at the distal (sometimes called vegetative) pole. The inner sheath is also lost, and the young Hydra fixes itself and begins to live as its parent or parents did. Forms like Hydra. — Even simpler than Hydra is Protohydra, without tentacles, occurring both in the sea and in fresh water. An American fresh-water form {Microhydra ryderi) is known to liberate free- swimming medusoids {Limnocodium) which have been found in Europe, e.g. in the Victoria Regia tanks in the Botanic Gardens, Regent's Park, London. Another species, L. kawaii, has been found in the J angtszekiang in China, 1000 miles from its mouth. A related form, Limnocnida, occurs in Lakes Tanganyika and Victoria Nyanza, and in the river Niger. A strange simple polyp — Polypodium — has been found as a parasite on the eggs of sturgeons. Further details in regard to all these forms are much wanted. Second Type of Ccelentera. — A Medusoid. Class Hydrozoa Hydra is too simple to be thoroughly typical of the Hydrozoa. The class includes the hydroid colonies or zoophytes, which may be compared to Hydrce with many buds, and also free medusoid forms, which may be (a) liberated members of a hydroid colony, or {b) independent organisms. Besides these there are complex colonies of medusoid forms (Siphonophora). The hydroid type, except in minor details, usually resembles Hydra. In some cases the tentacles are solid, HYDROIDS AND MEDUSOIDS l8l instead of hollow as in Hydra, and they may be arranged in two circles — an outer and an inner {e.g. Tubularia). In some of the hydroid colonies, notably the Millepores and Hydr actinia, the polyps are very dissimilar to one another, and have become specialised for the performance of different functions. The medusoid type is like an inflated hydroid adapted Fig. 90. — Bougainvillea. — Alter AUman. A. A small piece of a hydroid colony, p., Perisarc ; m., medusoid bud ; h., hydranth or polyp head. B. A medusoid. ma., Manubrium; r.c, radial canal; s., sense organ. for swimming. It is bell-shaped, and down the middle of the bell hangs a prolongation — the manubrium — which terminates in the mouth. Around the margin of the bell there is a little shelf, the velum or craspedon, which projects inwards, and is furnished with muscle cells. The margin of the bell also bears tentacles, usually hollow, and abundantly furnished with stinging cells (Fig. 83, 3). l82 PHYLUM CCELENTERA On the convex surface of the hell the ectoderm forms simply an epithelial layer ; on the concave surface it is differentiated into muscle cells on the velum, the manu- brium, and the tentacles, nerve cells at the base of the velum, and stinging cells on the tentacles. The endoderm Fig. 91. — Structiire of a Swimming-bell or Medusoid, Obelia geniculata, budded off from a Campanularian Hydroid. M., Mouth on the short manubrium ; R.C., one of the four radial canals from the central stomach to the circumference canal (C.C.) round the margin ; G., gonad on radial canal (Leptomedusoid) ; T., numerous marginal tentacles, which have small internally projecting vesicles (.S.) at their base. These are not to be confused with 8 minute spherical balancing organs or statocj'sts (ST.), situated adradially on the margin. is ciliated ; it lines the food canal, and extends also into the tentacles. The mesoglcea forms a thickened jelly, present more especially on the convex (ex-umbrellar) surface. The mouth opens into the canal of the manubrium, which leads to the central cavity of the dome. With this a varying number of unbranched radial canals communicate ; these open into a marginal circular vessel, which communicates STRUCTURE OF MEDUSOID 183 with the cavities of the tentacles. A plate of endoderm lies in the mesoglcea between the radial canals. Digestion is intracellular, and probably goes on throughout the whole of this " gastro-vascular " system. The movements of the bell are caused by the contrac- tions of the ectodermic muscle cells. The nervous system consists of a double ring of nerve fibres around the margin of the bell. With these are associ- ated ganglionic cells, which apparently control the muscular contractions. Sense organs may be present, in the form of " eyes," at the base of the tentacles (Ocellatae), or in the form of " auditory " (probably balancing) vesicles developed as pits in the velum (Vesiculatae). The reproductive organs develop either in the manu- brium or on the radial canals. The products always (?) ripen in the ectoderm, and often seem to arise there ; but Weismann and others have shown that the reproductive cells of a medusoid derived from a hydroid, or of the reduced and fixed repro- ductive persons in many hydroids, have considerable powers of migration, and may originate (sometimes in the endoderm) in the hydroid colony at some distance from the place where they are matured within the medusoid bud. The sexes are usually separate. The commonest kind of free-swimming larva is the planula, which is oval, ciliated, and diploblastic, devoid of an open- ing, and usually without a central cavity. In the case of those medusoids which arise as liberated sexual members of a fixed asexual hydroid colony, the planula settles down, loses its cilia, buds out tentacles, and develops into a new hydroid. In many hydroid colonies, as has been already noticed, Fig. 92. — Structure of a Medu- soid. — After Allman. ST., Stomach ; M., manubrium ; V., velum; T., tentacle; C.C., circum- ference canal ; G., gonad ; R.C., radial canal ; EN., endoderm ; EC, ectoderm ; MG., mesoglcea. 184 PHYLUM CCELENTEKA the sexual members are not set free, but remain as buds attached to the parent. These fixed " gonophores " show many stages of degeneration ; some, notably in the floating colonies of Siphonophora, differ little structurally from true medusoids, while others, as in Hydr actinia, are simply small closed sacs enclosing the genital products (Fig. 84). Third Type of Ccelentera. — The common Jelly-fish — Aurelia aurita. Class ScYPHOMEDUSi^ This Medusa is almost cosmopolitan, and in the summer months occurs abundantly around the British coasts. It swims by pulsating its disc, and also drifts along at rest without any pulsations. They often occur in great shoals, and hundreds may be seen stranded on a small area of flat sandy beach. The glassy disc usually measures about four inches in diameter, but may be twice as large. The jelly- fish feeds on small animals, such as copepod crustaceans, which are entangled and stung to death by the long lips. External appearance. — The animal consists of a gela- tinous disc, slightly convex on its upper (ex-umbrellar) surface, and bearing on the centre of the other (sub- umbrellar) surface a four-cornered mouth, with four long much-frilled lips. The circumference of the disc is fringed by numerous short hollow tentacles, by little lappets, and by a continuation of the sub-umbrella forming a delicate flap or velarium. Conspicuously bright are the four re- productive organs, which lie towards the under surface. Nor is it difficult to see the numerous canals which radiate from the central stomach across the disc, the eight marginal sense organs, and the muscle strands on the lower surface (Fig- 93)- The three layers. — ^The ectoderm which covers the external surface bears stinging cells, sensory and nerve cells, and muscle cells. The ectoderm seems also to be invagin- ated to form the gullet or stomodaeum. The endoderm lines the digestive cavity, is continued out into its radiating canals, and is ciliated throughout. The mesoglcea is a gelatinous coagulation containing wandering amoeboid cells froin the endoderm. The whole animal is very watery ; indeed, the solid parts amount to not more than 10 per STRUCTURE OF JELLY-FISH i8s cent, of the total weight. Yet some jelly-fish (species of Rhopilema) are used as food in Japan ! Nervous system. — The nervous system consists — {a) of a special area of nervous epithelium, associated with each of the eight sense organs, and {h) of numerous much-elongated bipolar ganglion cells lying beneath the epithelium on the under surface of the disc. This condition should be con- trasted with the double nerve-ring in Craspedote medu- soids, but too much must not be made of the contrast, for a nerve-ring is described in Cubo-medusae, one of the orders of Acraspedote jelly-fish. In Aurelia the sense organs are less diff^erentiated than in many other jelly-fish. Each of the eight organs, protected in a marginal niche, consists of a pig- mented spot, a club-shaped projection with numerous calcareous " otoliths " in its cells, and a couple of apparently sensitive pits or grooves. The sense organs arise as modifications of tentacles, and are often called " tentaculocysts " or " rhopalia." The.r ^^ ^-ZllI^rRotrl''"'*'' cavities are in tree com- munication with branches of the radial canals. Muscular system. — Between the plexus of nerve cells and the sub-umbrellar mesogloea there are cross-striped muscle fibres, each of which has a large portion of non-contractile cell sub- stance attached to it. They lie in ring-like bundles, and by their contractions the medusa moves. Unstriped muscle fibres are found about the tentacles and lips. Alimentary system. — The four corners of the mouth are extended as four much-frilled lips, each with a ciliated groove and stinging cells, and with an axis of mesogloea. They exhibit considerable mobility. Their crumpled and mobile bases surround and almost conceal the mouth. A Showing four genita pockets in centre, much -branched radial canals, eight peri- pheral niches for sense organs, and peri- pheral tentacles. 1 86 PHYLUM CCELENTERA short gullet or *' manubrium " connects the mouth with the digestive cavity in the centre of the disc. From this central chamber sixteen gastro-vascular canals of approximately equal calibre radiate to the circumference, where they open into a circular canal, with which the hollow tentacles are connected. Eight of the radial canals are straight, but the other eight are branched, and thus in an adult Aurelia the total number of canals is large. These canals are really due to a partial obliteration of the gastric cavity by a fusion of its ex-umbrellar and sub-umbrellar walls along definite lines. They are all lined by ciliated endoderm. Where the gullet passes into the central digestive cavity, there are four strong pillars of thickened sub- umbrellar material. Be- side these pillars, there are four patches where the sub-umbrellar sur- face remains thin. These are the gastro - genital membranes, lined in- ternally by germinal epithelium (Fig. 94, R.). To the inside of these genital organs, within the digestive cavity, are four groups of mobile gastric filaments {g.f., Fig. 94), which are very characteristic of jelly-fish. In appear- ance these are very similar to the small tentacles of the margin, and, like them, are hollow. They are covered with endoderm — with ciliated, glandular, muscular, and stinging cells. The body is mapped out into regions by the following convention : The first tentacles to appear in the larva are four in number, and correspond to the four angles of the mouth ; the radii on which they appear are called " perradial," marked by the four lips. Half-way between these, four " interradials " are then developed, marked by the gonads and gastric filaments. Then eight " adradials " may follow, between perradii and interradii, marked by the eight unbranched radial canals. Reproductive system. — The sexes are separate. The reproductive organs — ovaries or testes — consist of plaited Fig. 94. -Vertical section of Aurelia. — After Claus. m.. Mouth ; st., stomach ; r.c, radial canal ; R., reproductive organs ; g.f., gastric fila- ments ; g.p., sub-genital cavity ; /., marginal tentacle ; s., sense organ ; the shaded part is mesogloea. STRUCTURE OF MEDUSA 1 87 ridges of germinal epithelium, situated on the four patches already mentioned, within sacs which are derived from and communicate with the floor of the gastric cavity. They are of a reddish violet colour, and at first of a horseshoe shape, with the closed part of the curve directed outwards. Afterwards the ridges become circular, and surround the walls of the sacs in which they lie. But the sub-umbrellar surface is modified beneath each genital sac in such a way that the sac comes to lie in a sub-genital cavity com- municating with the exterior {g.p.^ Fig. 94). The con- tractions of the umbrella produce a rhythmic movement of the water which enters the sub-genital cavities, and this constant renewal of the water suggests some respiratory significance for the sacs. The genital sacs containing the plaited ridges of germinal epithelium communicate with the gastric cavity only, while the sub-genital cavities con- taining water and enveloping the genital sacs communicate with the exterior only. The ova and spermatozoa pass from the frills of germinal epithelium into the sacs, and thence into the gastric cavity. They find exit by the mouth, but young embryos may be found swimming in the gastro-vascular canals, and also within the shelter of the long lips. Variations. — The jelly-fish often exhibits variations, i.e. inborn changes of germinal origin which result in the organism being different from the norm or average of its species. It is normally tetrapartite, but sexpartite, penta- partite, and, more rarely, tripartite forms occur ; and the detailed variations are manifold. Life-history. — The fertilised ovum divides completely, but not quite equally, to form a blastula, with a very narrow slit -like cavity. From the larger-celled hemisphere, single cells migrate into the cavity, and fill it up with solid endoderm. The archenteron or primitive gut cavity arises as a central cleft in this cell mass, and opens to the exterior temporarily by the primitive mouth. During these processes the embryo elongates, the outer cells become ciliated, and the mouth closes. Thus the embryo becomes a free-swimming oval planula. After a short period of free lif% this planula settles down on a stone or seaweed, attaching itself by the pole where the mouth formerly opened. At a very early stage the mesogloea appears between the two layers. At the free pole an ectodermic invagination next occurs, an opening breaks through at its lower end, and thus a gullet lined with ectoderm is formed, which hangs freely in the general cavity. During 1 88 PHYLUM CCELENTERA this process there are formed first two and then four diverticula of the general cavity, which are arranged round the gullet above, and open freely into the digestive cavity below. In the gullet region these are separated by broad septa, which are continued into the lower region of the body as four interradial ridges or taeniolae. The tentacles bud out from the region of the mouth, the first four corresponding in position to the four pouches. Interradially above the four septa, four narrow funnel-shaped invaginations arise ; these are produced by the ingrowth of ectoderm, which then forms the muscle fibres which run down the Fig. 95. — Diagram of life-history of Aurelia. — After Haeckel. I, Free-swimming embryo ; 2-6, various stages of Hydra-tuba ; 7. 8, Strobila stage ; 9, liberation of Ephytje ; 10, 1 1 growth of Ephyrae into Medusae. taeniolee (contrast the endodermic muscles of Anthozoa). In contrasting this development with that of the hydroid polyp, Goette specially emphasises the fact that the radial symmetry is first indicated by the gut pockets, and the tentacles are late in development. Goette describes a quite similar process of development in certain sea- anemones, and claims to have found there rudiments of septal pockets and ectodermal muscles, thus confirming his view of the intimate relation between the Anthozoa and Scyphomedusae. The larva now forms a " Hydra-tuba " or " Scyphistoma " , it is about an eighth of an inch in height. By lateral budding, or by the formation of creeping stolons, it may give rise to larvas like itself. DEVELOPMENT OF AURELIA 189 The gradual widening of the central cavity renders the gullet tube less obvious, and results in an increasing resemblance to the medusa type. In late autumn, however, a more fundamental change occurs in the history of the Hydra-tuba, (a) Occasionally, by a " telescoping," the Scyphistoma becomes detached ^M° Fig. 96. —Lucernaria. — After Korotneff. and converted into a free-swim- ming Eph>T:a, which in turn be- comes a jelly-fish, {b) Sometimes, in unfavourable conditions, a furrow appears roimd the upper region of the Scyphistoma, the upper portion is converted into an Ephyra, and floats away, while the lower portion re-forms its oral region by regeneration, and pro- duces another Ephyra. (c) In ordinary conditions the Scyphi- stoma elongates, and displays a succession of annular constric- tions. This stage, often compared to a pUe of discs or saucers, is called a Strobila. Each disc is separated off in its turn as a free-swimming Ephyra, which becomes a jelly-fish. The still undivided basal portion may rest for a time, and then undergo further constric- tion. This is probably an abbreviation of the primitive mode of de- velopment. In the conversion of the Scyphistoma into the Eph\T:se, the diverticula coalesce into a general cavity, the entrances to the septal invaginations probably persist as the sub -genital pits, the gastric filaments sprout out from the remains of the septa, and so mark the place where the ecto- dermal gullet passed into the endodermal cavity. The first Ephyra differs from those which come after it in bearing the original tentacles of the Hydra-tuba. From its margin eight bifid lobes grow out, each embracing the base of a perradial or interradial tentacle. The bases of these eight tentacles become the sense organs or rhopalia. The other eight adradial tentacles atrophy. On the Ephyrae which follow there are at first no tentacles, only the eight bifid marginal lobes which bear the sense organs in their niches. Fig. 97. -Diagram of Lucernaria. — After AUman. C, Cavity of gut (ccelenteron) ; F., gastric fila- ments ; H., hypostome ; G., gonad ; T., tentacle ; C.C., circumference canal. 1 90 PHYLUM CCELENTERA This development illustrates alternation of generations. From the fertilised ovum a fixed asexual Scyphistoma results. This grows into a Strobila, from which transverse buds or Ephyrae are liberated. Each of these grows into a sexual jelly-fish, producing ova or spermatozoa. Relatives of Aurelia. — The Medusa?, or true jelly-fish, include forms which agree with the Anthozoa in relative complexity of structure as compared with Hydrozoa, and in the possession of an ectodermal gullet, but differ in possessing ectodermal septal muscles and in some histological features. If Goette's discovery of rudimentary ectodermal muscles in the larvae of certain sea-anemones be confirmed, however, it would greatly increase the probability of a close relationship between the two sets. Among the Scyphomedusae closely allied to Aurelia some, e.g. Pelagia, have a direct development without the intervention of Scyphistoma or Strobila stages, but this may occur exceptionally in Aurelia. Cyanea is often very large, " it may measure j}, ft. across the bell, with tentacles 120 ft. long." Chrysaora is hermaphrodite, and has diffuse sperm sacs even upon the arms. In the Rhizostomae, e.g. Cassiopeia and Pilema, the mouth is obliterated, and replaced by numerous small pores on the four double arms. Lucernaria and its allies are interesting sessile forms which have been compared to sexual Scyphistomas, that is, are regarded as persistently larval forms (Figs. 96 and 97). Contrast between Medusoids {Hydromedusce) and Medusce {Scyphomediisce) Medusoids. (Craspedota.) The majority are small " swimming- bells." A flap or velum (craspedoii) projects inwards from the margin of the bell. No taeniolcB, nor gastric filaments. A double nerve-ring around the margin . Naked sense organs either optic or "auditory." They are usually derived from the skin, but the " auditory " sacs may be modified tentacles. Reproductive organs on the radial canals or by the side of the manubrium. The reproductive cells are usually derived from the ectoderm. With the exception of the Trachy- medusae, all arise as the liberated reproductive personae of liydroid colonies. {N.B. — "Auditory" organs are prob- ably for balancing or equilibration.) Medusa. (Acraspeda.) Many are large " jelly-fish." No velum. (The velarium of Aurelia is a mere fringe, very inconspicuous in tlie adult, and not inturned.) In the Scyphistoma there are four taeniolffi, from part of which the gastric filaments of the adult grow. Eight separate nervous centres beside the sense organs, and a sub-umbrellar nervous plexus. Sense organs are modified tentacles, and probably have almost always a triple function. They are usually protected by a hood. Reproductiv'e organs in special pockets on the floor of the gastric cavity. The reproductive cells arise in the endoderm. Have no connection with hydroids, but may have a small sedentary polyp- stage (or Scyphistoma) in the course of their life-history. Probably more nearly related to Anthozoa than to Hydrozoa. SEA- ANEMONES IQI Fourth Type of Ccelentera. — A Sea-Anemone, such as Tealia eras sic or tits. Class Anthozoa Most sea-anemones live fixed to the rocks about low- water mark. All these fixed forms have a distinct basal disc, and may, like Tealia crassicornis, be half buried in sand and gravel ; others, without a basal disc, are loosely inserted in the sand, e.g. Edwardsia and Cerianthiis. All are able to shift their positions by short stages. Some \iV^ /A Fig. 98. — External appearance of Tealia crassicornis. reef-anemones {Cradactis) can crawl about on their tentacles. They feed on small animals — molluscs, crustaceans, worms — which are caught and stung by the tentacles. Many depend on minute organisms ; others may be seen trying to engulf molluscs decidedly too large for them. A few anemones, without pigment or with little, have symbiotic Algae in their endoderm cells ; the bright pigments of many others seem to help in respiration. Besides the sexual reproduction (in which the young are sometimes developed within the parent), some sea-anemones also multiply asexually by detaching portions from near the base, and fission occurs in a few forms. 192 PHVLtJM CCELENTERA External appearance of a fixed Anemone. ^ — The cylindrical body is fixed by a broad base ; it bears whorls of hollow tentacles around the oral disc ; the mouth is usually a longitudinal slit. The tentacles are contracted when the animal is irritated, and the whole body can be much reduced in size. Just below the margin of the oral disc there is a powerful sphincter muscle ; this contracts, and pulls together the body wall over the mouth and retracted tentacles. Water may pass out gently or other- wise by a pore at the tip of each tentacle, and long white threads, richly covered with stinging cells, can be ejected in many anemones through the walls of the body (Fig. 99)- General structure.- — The Anthozoon polyp differs markedly from the Hydroid polyp — not only because an in- vagination from the oral disc inwards has formed a gullet tube, which hangs down into the Fig. 99. — Vertical section of a sea- anemone. — After Andres. /., Tentacles ; o., mouth ; oes., oesophagus ; c, c'., apertures through a mesentery ; a., a'., acontia ; g., genital organs on mesentery ; m.f., mesenteric filaments ; jn.l., longitudinal muscles ; s., primary septum or mesentery ; tCbasarS'"^^'™' ^"•' t^^t^^-^y ^^Pt""^ ; general cavity, but also because a number of partitions or mesenteries extend from the body w^all towards this gullet. Some of the partitions are " com- plete," i.e. they reach the gullet ; others are " incomplete," i.e. do not extend so far inwards. The complete mesenteries are attached to the oral disc above, to the side of the gullet, and to the base, and all the mesenteries are ingrowths of the body wall. The cavity of the anemone STRUCTURE OF SEA-ANEMONE 193 mf ml is thus divided into a number (some multiple of six) of radial chambers. These are in communication at the base, so that food particles from the gullet may pass into any of the chambers between the partitions. Moreover, each partition is perforated, not far from the mouth, by a pore, besides which there is often another nearer the body wall. The tentacles are continuous with the cavities between the mesenteries, and thus all the parts of the body are in communication. The mouth is usually a longi- a tudinal slit, and its two corners are often richly ciliated. The gullet is marked with longitudinal grooves, two of which, the *' siphonoglyphs," correspond to the corners of the mouth, and are especially broad and deep. Along these two grooves, and by these two corners, food particles usually pass in ; but in some, one side is an incurrent, the other an excurrent channel. Occasionally only one corner of the mouth and side of the gullet is thus modified. The gullet often extends far down into the cavity of the anemone. It admits of a certain amount of extrusion. The mesen- teries bear — {a) mesenteric filaments ; [h] retractor muscles ; {c) ridges of reproductive cells, almost always either ova or spermatozoa, rarely both ; and {d) in some cases oflfensive threads or acontia. The mesenteric filaments seem to be closely applied to the food, and perhaps secrete digest- ive ferments. Intracellular digestion also occurs. Sea- anemones have no sense organs ; the sapphire beads, which are so well seen at the bases of the outermost tentacles of the common Actinia mesembryanthemum, are batteries of 13 Fig. 100. — Section through sea- anemone (across arrow in Figure 99). — After Andres. A, B, Directive septa; m.f., mesenteric filaments ; g., genital organs ; m.L, longitudinal muscles ; s., primary sep- tum; s'., secondary septum; s"., tertiary septum. The arrow enters between two primary septa (an intraseptal cavity), and passes out between two tertiary septa. 194 PHYLUM CCELENTERA Stinging cells. The nervous system Is iincentralised, and consists of superficial sensory cells connected with a plexus of sub-epithelial ganglion cells. The layers of the body. — The ectoderm which clothes the exterior is continued down the inside of the gullet. The endoderm lines the whole of the internal cavity, including mesenteries and tentacles. The mesogloea is a supporting plate between these two layers, and forms a basis for their cells. The ectoderm consists of ciliated, sensory, stinging, and glandular cells, and also of sub-epithelial muscle and ganglion cells based on the mesogloea. but mainly restricted to the circumoral region. The endoderm consists mainly of flagellate cells, with muscle fibres at their roots. These form the chief muscle bands of the wall, the s s Z A Fig. ioi. — Z, Diagrammatic section of Zoantharian ; A, of Alcyonarian. — After Chun. The line S-S in Z is through the siphonoglyphs (a), the line T-T passes through two inter-septal spaces. The retractor muscles are represented by dark thickenings on the mesen- teries — all on one (the ventral) side in the Alcyonarian. The line S-S in A represents the axis of symmetry. mesenteries, and the gullet. Nor are glandular and even nerve cells wanting in the endoderm. The mesenteries. — In sea-anemones and nearly related Anthozoa, twelve primary mesenteries are first formed. These are grouped in pairs, and the cavity between the members of a pair is called intra- septal, in contrast to the inter-septal cavities between adjacent pairs. In these inter-septal chambers other mesenteries afterwards appear in pairs. Two pairs of mesenteries, however, differ from all the rest — those, namely, which are attached to the two corners of the mouth and to the corresponding grooves of the gullet. These two pairs of mesenteries are called "directive," and they divide the animal into bilaterally sym- metrical halves. Anatomically, a pair of directive mesenteries differs from the other paired mesenteries, because the retractor muscles, which extend in a vertical ridge along them, are turned away from one another, and run on the inter-septal surfaces, whereas in the other mesenteries SEA-ANEMONES 195 the retractor muscles run on the intra-septal surface — those of a pair facing one another. The arrangement of these muscles is of great im- portance in classifying Anthozoa. It is possible that the mesenteries are homologous with the tasniolae of jelly-fish, and the mesenteric with the gastric filaments. But some embryologists maintain that the mesenteric filaments are derived from the ectoderm of the gullet. From the above description it will be noticed that the fundamental radial symmetry of the Coelentera has here become profoundly modified. Development. — From the fertilised ovum a blastula may result which by invagination becomes a gastrula. In some cases the ovum segments into a solid morula ; this becomes a free planula, in which a cylindrical depression at one pole forms the mouth and gullet. Or the two layers may be estabhshed by a process known as delamination, in which a single layer of cells is divided into an inner endodermic and an outer ectodermic layer. The planula settles down by the aboral pole, and develops like a Hydra-tuba. The larva of Cerianthids is for a time pelagic, and used to be recognised as a distinct genus, Arachnactis. Related forms. — The sea-anemones are classified in the sub-class Anthozoa or Actinozoa, and along with many corals are distinguished as Zoantharia or Hexacoralla from the Alcyonaria or Octocoralla, like A Icyonium. Anthozoa or Actinozoa Zoantharia, Hexacoralla, e.g. Sea-Anemone. Many are simple, many colonial. The polyps of a colony may give rise to others directly by fission or budding. Tentacles usually simple, usually some multiple of six, often dissimilar. Mesenteries usually some multiple of six, complete and incomplete. Retractor muscles never as in Alcyo- naria. Two gullet grooves or siphonoglyphs, or only one. No dimorphism. Calcareous skeleton, if present, is derived from the basal ectoderm. Examples. Sea - anemones — e.g. Tealia and Actinia. Madrepore corals, many of them reef- building. Antipatharians. An aberrant Anti- patharian, Dendrobrachia fallax, has eight feathered tentacles. Alcyonaria, Octocoralla, e.g. Dead-Men's-Fingers. All colonial, except a small family including Monoxenia and Haimea. The polyps of a colony give rise to others not directly, but through stolons or solenia. Tentacles eight, feathered, uniform. Mesenteries eight, complete. Retractor muscles always on one (ven- tral) side of each mesentery (see Fig. loi). One (ventral) gullet groove (siphono- glyph or sulcus), or none. Frequent dimorphism among members of a colony. There are usually calcareous spicules (of ectodermic origin) in the mesogloea. Examples. Alcyonium (Dead-men's-fingers), with diffuse spicules of lime. Tiibipora (Organ - pipe coral), with spicules fused into tubes and trans- verse platforms. Coraliium rubrum (Red coral), with an axis of fused spicules. Pennatula (Sea-pen), a free phosphor- escent colony, with a " horny " axis, possibly endodermic. 196 PHYLUM CCELENTERA ZOANTHARIA The Zoantharia include many orders, e.g. the primi- tive Cerianthidea {Cerianthus, etc.) and Edwardsiidea (Edwardsta), the Actiniidea (including the typical sea- anemones and the Madreporaria), and the divergent Anti- pathidea. Making of a typical coral. — Although the term Fig. 102. — The formation of a coral shell {Astroides). — After Pfurtscheller. St., StomodiBum ; ms., mesentery ; s., calcareous septum ; B., basal plate. " coral " is applied to many different Coelenterate types with substantial calcareous skeletons, e.g. to Millepores which are Hydrozoa, and to " blue corals " and " red corals " which are Alcyonarians, the corals par excellence are the Aladreporarians. They form the coral rock and " coral islands " found in many parts of the globe, but rarely north or south of a belt extending 30° on each side of the equator, and rarely below the 40-fathom line. In a general way a Madrepore polyp is like a sea-anemone in structure, and the " coral " it forms is its external shell ZOANTHARIA 197 rather than its skeleton. It is altogether a product of the ectoderm. From one polyp others usually arise by budding or by division, e.g. Astrcea and Madrepora and Lophohelia (North Sea), but there are solitary forms such as Fungia and Caryophyllia (British). The first part of the " shell " to be formed is the basal plate between the ectoderm of the base and the substratum. FiG. 103. — Structure of Antipatharians. 1. A group of polyps — M., mouth ; t., tentacles. 2. Axis without polyps and ccenenchyma, covered with spines [Sp.). 3. Vertical section of a polyp — ^., axis; ^, tentacle; g., gullet; m., mesentery ; 0., ovary ; m.f., mesenteric filaments. 4. Cross-section of a polyp — EC, ectoderm; M., mesogloea ; EN., endoderm ; G., gullet ; MS., mesenteries. On this plate a number of radially arranged vertical ridges (septa or cnemes) are then formed, and as they grow in height they push the ectoderm of the base up before them (see Fig. 102). An external wall or theca is then formed, partly by the fusion of the outer margins of the septa and partly by a circular upgrowth from the basal plate. This theca pushes the body wall before it, as the septa pushed the base. Sometimes a second external wall or epitheca is formed outside of and concentric with the theca. By the 198 PHYLUM CGELENTERA coalescence of septa in the central line a columella or median pillar may be formed. The outer wall of the theca may bear vertical ridges or costae, and these may be connected with neighbouring costae of other polyps by horizontal shelves or dissepiments. Both septa and costae correspond to intermesenteric spaces. Antipatharians. — Usually arborescent, some- times whip-like colonies, of wide distribution, often called " black corals." A spinose hollow MMWis tmtft I. M. \J Fig. 104. — Diagrams of Types of Alcyonaria. — After Hickson. Types of Alcyonaria : — I. Of Stolonifera ; II. of Alcyonacea ; III. of Axifera; IV. of Stelechotokea, horny axis is covered with coenenchyma and regularly arranged polyps, without any trace of spicules. A polyp is usually oval in'section, with its long diameter in the line of the axis, and its gullet ALCYONARIANS 199 elongated at right angles to this. There are usually six simple non- retractile tentacles, 6-10 mesenteries, and two ill-defined siphonoglyphs. The mesenteries are without muscle-banners. The two longest, running at right angles to the elongated stoniodacum, bear gonads. The development is unknown. Examples : — Antipatkes (arborescent). Cirripaihcs (whip-like). Lciopaihes (with twelve mesenteries). Dendruhrachia (with eight pinnate re- tractile tentacles). Alcyonaria In the Alcyonarian polyp there are eight tentacles, almost always pinnate, and eight mesenteries attached to -rp Fig. 105. — Corallium rubrum, a corner of a colony. — After Lacaze-Duthiers. A., Anthocodia or retractile portion^ of a polyp; r.p., com- pletely retracted polyp, with the verruca or calyx portion left protruding ; C., ccenenchyma ; T., pinnate tentacles. the stomodaeum. In Bourne's Acrossota the tentacles have no pinnules. There is one longitudinal ciliated groove (siphonoglyph or sulcus) in the stomoda?um (ven- trally). The mesenteries bear retractor muscles, all situated on the sulcar aspect (see Fig. loi), and each mesen- tery bears a mesenteric filament. The two dorsal (asulcar) 200 PHYLUM CCELENTERA mesenteries are long, ciliated, and non-glandular ; they are respiratory in function and cause an upward current, that in the sulcus being downward. Many Alcyonarians are dimorphic, having in addition to the typical polyps {autozooids) dwarf siphonozooids , with suppressed tentacles, strongly developed sulcus, no mesenteric filaments, and often ill-developed mesenteries. Their function is to drive currents of water through the canal systems of the colony, and they are sometimes reproductive as well. With the exception of one small family of solitary forms Fig. io6.— Alcyonarian spicules. (Haimeidae), the Alcyonarians form colonies which are in various ways supported by spicules, or by spicules and an axis. The spicules, which take the most diverse forms, seem to be begun at least by ectodermic cells (a pair to each spicule), iDut they usually pass into the mesogloea. The nematocysts are usually small. A number of Alcyon- arians are viviparous ; the embryo is usually a planula. Colonies are formed in different ways, (i) A parent polyp gives off hollow stolons or solenia, which bud off new polyps, and the whole forms a spreading network or fiat plate, e.g. Clavularia, a type of Stolonifera (Fig. 104, I.). (2) The polyps may be crowded together so as to form bundles raised on a stalk, or lobose fleshy growths with the polyps projecting on the surface of a dense mesogloeal mass honeycombed by solenia, e.g. Xenia and Alcyonmm, types of Alcyonacea (Fig. 104, II.). CLASSES OF CCELENTERA 201 (3) Or the colony may raise itself in the water by forming a common upright coenenchyma, in which the polyps are embedded, and the medullary part of which may form a substantial axis of cemented spicules, e.g. Corallium, a type of Pseudaxonia. (4) Or tiie vertical extension of the colony may be effected by a horny secretion from the polyps, which comes to form an axis, really outside of the polyps though encrusted by them. This axis may be purely horny or in part calcareous, e.g. Gorgonia and Acanella, types of Axifera (Fig. 104, III.). (5) Fifthly, the vertical extension may be due to a great elongation of a single primary polyp which gives off solenia bearing numerous secondary polyps, e.g. Pennatida, a type of Stelechotokea (cf. Fig. 104, IV.). An altogether aberrant type is represented by the blue coral {Heliopora) and its extinct relatives {Heliolites, etc.). General Survey of Ccelentera Before we proceed to the systematic survey, we may contrast the essential structural features of the four classes of Ccelentera. I. In the Hydrozoa or Hydromedusae there is no inturned ectodermic gullet or stomodaBum ; there are no partitions or mesenteries ; there are no special digestive organs ; in the body wall the ectodermic muscles are mostly longitudinal and the endodermic muscles circular ; the sex cells are usually produced in the ectoderm ; there is very frequentlv a combination of polypoid and medusoid phases in the life- history ; the circumference of the medusoid bears a muscular velum of ectoderm and mesogloea ; there is no calcareous secretion (except in Millepores). II. In the Scyphomedusae there is an inturned ectodermic gullet or StomodaBum ; there are hints of mesenteries ; there are special digestive filaments ; the sex cells are endodermic ; there is no velum ; there is often a non-sexual sedentary stage ; there is no calcareous secretion. III. In the Anthozoa there is an intiurned ectodermic gullet or stomodffium ; there are distinct mesenteries or partitions from body wall to gullet wall ; there are often digestive filaments ; in the body wall the ectodermic muscles are circular (except in Cerianthus), and the endodermic muscles longitudinal ; the sex cells are endodermic ; there is no medusoid phase. IV. The Ctenophora are very divergent and apart from the other classes, e.g. in rarely having any stinging cells, and in having a well- defined mesoderm. SYSTEMATIC SURVEY Class I. Hydrozoa Solitary polyps hke Hydra, hydroid colonies or zoophytes with medusoid reproductive buds, medusoids without sedentary stages, colonies of modified medusoids. 202 PHYLUM CCELENTERA I. Order Hydromedusae.— Simple or colonial forms in which the sexually reproductive persons are either liberated as free-swimming medusoids or are sessile gonophores. (a) Hydrophora.— Two types are included here. The first mcludes the Tubularians, Hydractinia, and other forms in which the polyps are not enclosed in the protective perisarc which often surrounds the colony (gymnoblastic), and in which the free medusoid forms, when present, have Fig. 107. — Diagram of a gymno- blastic Hydroid. — After All- man. a. Stem ; b, root ; c, gut cavity ; d, endodcrni (dark) ; e, ectoderm ; /, horny perisarc ; g, hydra - like " person " (hydranth) ; g', the same, contracted ; h, hypostome bearing mouth ; k, sac-like repro- (hictivebud (sporosac) ; /.medusoid " person " ; m, a modified hydranth (blastostyle) bearing sporosacs. I. i Fig. 108. — Graptolites. I. Monograptus. II. Diplograptus. their genital organs placed in the wall of the manubrium (Antho- medusa;), and are furnished with ocelli placed at the base of the tentacles. Hydra and its allies may be included here. An unattached marine hydroid — Hypolytus peregrinus — has been described, and as it bore gonophores it was obviously mature, which is doubtful as regards two other unattached forms, Protohydra leuckartii HYDROID ZOOPHYTES 203 Fig. 109. — Hydroids. — After Hincks. Tubularia. II. A. Piece of Sertularia. II. B. A fragment enlarged, showing sessile hydrothecae (H.) on both sides of the twigs. III. A. Plumularia. III. B. A fragment enlarged, showing hydrothecae IH.) on one side of each twig, an axillary gonotheca (G.) and minute nemato- phores. IV. A. Campanularian. IV. B. A fragment enlarged, showing stalked hydrothecaB (H.), a gonotheca (G.) ; C, the ccenenchyma ; P., the perisarc ; S., a stalk. 204 PHYLUM CCELENTERA and Halermita cumulans, which may turn out to be larval. The hydroid stages of Pelagohydra and Margelopsis are free -swimming. Examples : — Syncoryne sarsii, the free medusoid of which is called Sarsm tubulosa. Bougainvillea ramosa liberates the medusoid Margelis ramosa. Cordylophora lacustris and T uhularia larynx have at- tached gonophores or sporosacs. The second type includes Campanularians and Ser- tularians along one line ; Halecids and Plumularians along another line. The protective perisarc sur- rounding the colony is con- tinued into little cups (hydrothecae) enclosing the polyps (calyptoblastic). These hydrothecae are stalked in Campanularians, sessile in Sertularians and Plumularians. The free medusoids have their gonads placed in the course of the radial canals (Lepto- medusae), and are either " ocellate " or " vesicu- late." Examples : — Plumularia, with hy- drotheca; on one side of the branches, and S ertul aria, with hydrothecae on both sides of the branches. The Campanularian Obelia geniculata liberates the medu- soid Obelia genicu- lata. {b) Hydrocorallinae. — Colonial forms which suggest the Hydractiniae in their polymorphism and division of labour, but are distinguished by their power of taking up lime, and so forming " corals." The colonies are com- plex and divergent, the reproductive persons are either sessile gonophores or simple medusoids. Millepora, Stylaster. (c) Trachymedusae. — These exist as a rule only in the medusoid form, and are divided into two groups. Trachomtdusa! and Narcomedusae, Fig. no. — Campanularian Hydroid. — After Allman. H., Hydrotheca or polyp-cup ; HY., hydranth or polyp-head ; G., gonotheca, enclosing a repro- ductive polyp producing medusoid buds ; M., a liberated medusoid ; ST., basal stolon. SURVEY OF CCELENTERA 205 according to the position of the gonads. Examples : Geryonia, Car- marina, Cunina, Aeginopsis. (Tlie fresh-water medusoid Limnocodiutn or Craspedacusta is budded oft" from the North American Microhydra ryderi). 2. Order Siphonophora. — Free-swimming colonies of modified medusoid persons (medusomes), with much division of labour. Physalia (Portuguese man-of-war), Diphyes, Velella, Porpita. Incertce sedis. Graptolites. — Extinct unattached colonies with a rod-like axis found in Upper Cambrian, Ordovician, and Silurian systems. The colony is usually linear, and consists of cup-shaped hydrothecae borne on one, two, or four sides of the solid axis {virgula). Each opens into a common median canal. At the proximal free end there is a minute triangular or dagger-shaped body — the sicula — which represents the embryonic skeleton. Some reproductive bodies or gonangia have been found. The animals were probably free- swimming in muddy seas, and of a Hydromedusan nature. Class II. ScYPHOMEDUS^ ( = Acraspcda) Jelly-fish with gastric filaments, sub-genital pits, and no velum — (i) Lucernariae. — Sedentary forms. Lucernaria, Haliclystus, and Depastrum. (2) Discomedusae. — Active forms, often with complicated life- history. Aurelia, Pelagia, Cyanea, Rhizostoma. (3) Cubomedusae. — Forms with broad pseudo-velum, and other peculiar features. Charybdea. (4) Peromedusse. — Forms with four inter -radial tentaculocysts only. Pericolpa. Class III. Anthozoa (=Actinozoa) Polypoid forms with well-developed gullet and septa, and circumoral tentacles, (i) Zoantharia or Hexacoralla. {a) Actiniaria. Sea-anemones. Actinia, Anemonia, Tealia, Cerianthus. {b) Madreporaria. Stone or reef corals. Asircsa, Madrepora, Fimgia, McBandrina. (f) Antipatharia. " Horny " black corals. Antipathes. (a) Alcyonaria or Octocoralla. Alcyonium (Dead-men's-fingers), Tubipora (Organ-pipe coral). Cor allium (Red coral)," Sea- fans, Pennatula (Sea- pen), Monoxenia (non-colonial). Class IV. Ctenophora Delicate free-swimming organisms, generally globular in form, moving by means of eight meridional rows of ciliated plates, or comb- like combinations of cilia. The stinging cells are almost always replaced by " adhesive cells." The mouth is at one pole, and leads 2o6 PHYLUM CCELENTERA into an ectodermic gullet. The gastric cavity is usually much branched. The mesodermic layer is well developed, and includes muscular and connective cells. At the aboral pole there is a sensory organ, including an " otolith," which seems of use in steering. Here, also, there are two excretory apertures. Except in Beroe and its near relatives, there are two retractile tentacles. All are hermaphrodite. The development is direct. They are pelagic, very active in habit, carnivorous in diet, and often phosphorescent. According to some, they lead on to Polyclad worms, especially through Ctenoplana and Coeloplana, two Fig. III. — Diagram of a Ctenophore. — After Chun. M., Mouth ; 5., sensory organ ; T., tentacle cut short ; SH., pouch of tentacle ; C, ciliated combs ; F., funnel or central canal ; SV., paragastric canal running parallel with stomodaeum ; G., other canals of the gut ; V., one of the meridional canals, bear- ing gonads, running under the bands of ciliated combs. curious flattened forms which crawl like Planarians. Mortensen's remarkable sessile Tjalfiella corroborates this affinit3\ Examples : — (a) With tentacles, Cydippe and the ribbon-shaped Venus' Girdle (Cestus veneris), (b) Without tentacles, Beroe. History of Coelentera. — Of corals, as we would expect, the rocks preserve a faithful record, and we know, for instance, that in the older (Palaeozoic) strata they were represented by many types. We often talk of the imperfection of the geological record, and rightly, for much of the library has been burned, many of the volumes are torn, whole chapters are wanting, and many pages are blurred. But this imperfect record sometimes surprises us, as in the quite distinct remains of ancient jelly-fish, which animals, as we know them now, are appar- ently little more than animated sea-water. We should also grasp the PEDIGREE OF CCELENTERA 207 conception, with which Lyell first impressed the world, of the uniformity of natural processes throughout the long history of the earth. Thus in connection with Coelentera we learn that there were great coral reefs in the incalculably distant past, just as there are coral reefs still. So in the Cambrian rocks, which are next to the oldest, there are on sandy slabs markings exactly like those which are now left for a few hours when a large jelly-fish stranded on the flat beach slowly melts away. On the other hand, some forms of life which lived long ago seem to have been very different from any that now remain, as is well shown by the abundant Graptolite fossils, which, though probably Coelentera, do not fit well into any of the modern classes. As to the pedigree of the Coelentera, the facts of individual life- history, and the scientific imagination of naturalists, help us to construct a genealogical tree — a hypothetical statement of the case. Thus it Fig. 112. — Hydroctena. A medusoid with suggestions of Ctenophore structure, but a true medusoid none the less. ab.o., Aboral sensory organ ; T., retractile tentacle ; v., velum ; M., month ; ST., stomach. seems very Ukely that the ancestral many-celled animals — ancestral to Sponges, Coelentera, and all the rest — were small two-layered tubular or oval forms. The many-celled animals must have begun as clusters of cells ; the question is, what sort of clusters — spheres of one layer of cells, or mouthless ovals, or little discs of cells, or two-layered thimble- like sacs ? Possibly there were many forms, but Haeckel and other naturalists were led to fix their attention especially on the two-layered sac or gastrula, because this form keeps continually cropping up as an embryonic stage in the life-history of animals, whether sponge or coral, earthworm or starfish, mollusc or even vertebrate, and also because this is virtually the form which is exhibited by the simx^lest sponges (Ascons), the simplest Coelentera {Hydra), and even by the simplest " worms " (Turbellarians). If we begin in our survey with such a gastrula-like ancestor, the probabilities are certainly in favour of the supposition that it was a free- 2o8 PHYLUM CCELENTERA swimming organism. A gradual perfecting of the locomotor character- istics might yield the two medusoid types of which we have already spoken. But we know that the common jelly-fish Aurelia has a prolonged larval stage which is sedentary, vegetative, and prone to bud. If we suppose with W. K. Brooks that many forms, less constitutionally active than others, relapsed into this sedentary state, with postponed sexuahty, and with a preponderant tendency to bud, we can understand how polyps arose, and these of two types, one nearer the jelly-fish and «! — J?> i-V. '-^t»- f :> » n -DK ' ■ Fig. 113. — Commensalism of sea-anemones and hermit-crab. Lucernarians and leading on to sea-anemones and corals, the other nearer the swimming-bell type and leading on to a terminus in Hydra. It is certainly suggestive that we have jelly-fish wholly free {Pelagia), jelly-fish with a sedentary larval life {Aurelia), jelly-fish predominantly passive (Lucernaria), and related polyps (Sea-anemones, etc.), which only occasionally rise into free activity ; while in the other series we have medusoid types always free (Trachymedusaa), others which are liberated from (Campauularian and Tubularian) sedentary hydroida, other (Sertularian and Plumularian) zoophytes whose buds though often medusoid-like are not set free, and finally Hydra, which, though it ECOLOGY OF CCELENTERA 209 may creep on its side, or walk on its head, is predominantly a sedentary animal, without any youthful free-swimming stage. Ecology. — The Coelentera are almost all marine. In fresh water we find the common Hydra, the minute Micro- hydra without tentacles, the strange Poly podium, which in early life is parasitic on sturgeons' eggs, the compound Cordylophora, occurring in canals and in brackish water, and the fresh-water Medusoids {Limnocodium and Limnocnida). Most of the active swimmers are pelagic, but there are also a few active forms in deep water. Many polyps anchor upon the shells of other animals, which they sometimes mask, and there are m.ost interesting constant partnerships between hermit-crabs and sea-anemones, e.g. between Eupagurus prideauxii and Adamsia palhata. The hermit-crab is masked by the sea-anemone, and may be protected by its stinging powers ; the sea-anemone is carried about by the hermit-crab, and may get crumbs from its abundantly supplied table. This illustrates a mutually beneficial external partnership or commensalism. In some other animals it may degenerate into parasitism (see Fig. Another kind of partnership is illustrated by many sea- anemones and Alcyonarians. Minute unicellular Algae (Zoochlorellae) live within the cells of the animals in close physiological partnership with them (symbiosis). A spatial partnership in which one animal finds habitual shelter within or near another is not infrequent; e.g. small horse-mackerels (Carangida;) swimming in shelter of large jelly-fish; a small fish [Amphiprion bicinctus) inside a giant sea-anemone (Crambactis arahica) which has a diameter of a foot ; another fish (Fierasfer) thai goes in and out of the hind-gut of Holothurians ; another that lives among the very long hair-like spines of the Red Sea rock-urchin {Diadema saxatile) ; and another {Apogonichthys strombi) that spends part of its time in the mantle cavity of the large sea-snail {Strombus gigas) of the Bahamas. The quaint little hydroid Lar sabellarum^Yives at the mouth of the tubes of the worm Sabella ; another hydroid {StylacHs minoi) grows all over the skin of a rock-perch [Minotis) from the Indian Ocean ; Stylactis vermicola was found on the back of the worm Aphrodite at the great depth of 2900 fathoms. M CHAPTER X UNSEGMENTED WORMS Phylum Platyhelminthes : Chief Classes — Turbellaria, Trematoda, Cestoda. Phylum Nemertea. Phylum Nemathelminthes : Chief Classes — -Nematoda, Nematomorpha, Acanthocephala. The title " worms " is hardly justifiable except as a con- venient name for a shape. The animals to which the name is applied form a heterogeneous mob, including about a dozen classes whose relationships are imperfectly known. It is likely that certain " worms " were the first animals definitely to abandon the more primitive radial symmetry, to begin moving with one part of the body always in front, to acquire head and sides. And if one end of the body constantly experienced the first impressions of external objects, it seems plausible that sensitive and nervous cells would be most developed in that much-stimulated, over- educated head region. But a brain arises from the insinking of ectodermic cells, and its beginning in the cerebral ganglion of the simplest " worms " is thus in part explained. Worm types begin the series of triploblastic coelomate animals, i.e. of those which have a well-defined mesoderm, and a coelom or body cavity lined with mesoderm and distinct from the gut. It must be noted, however, that the appearance of a well-developed ccelom and mesoderm is very gradual ; thus there is practically no coelom in the Platyhelminthes, and the mesoderm is sometimes not more definite than in Ctenophora. 2 lO FLAT-WORMS 211 Phylum Platyhelminthes The Platyhelminthes or flat-worms include three chief classes — Turhellarians^ Trematodes, and Cestodes — which form a related series. The body is flattened from above downwards ; the mesoderm forms a compact mass of cells or parenchyma without a definite coelom ; there is the be- FiG. 114. A. A minute portion of the branched excretory system of a Platyhehninth, showing longitudinal duct (I.), with cilia (C.), its branches (IL and IIL), and the terminal flame-cells (IV.). B. One of the characteristic hollow flame-cells, leading into the excretory tubule (i), showing the long cilia (2), the excretory globules (3), the nucleus (4), and pseudopodia-like processes (5) passing among adjacent cells. ginning of a head-brain ; the excretory system consists of a pair of lateral canals, giving off many branches, whose twigs end in peculiar " flame-cells " ; almost all are hermaphrodite. There is no doubt that the three classes, Turbellarians or Planarians, Trematodes or Flukes, and Cestodes or Tape- worms, are related to one another. A fourth class of Temnocephalids must also be admitted. It is interesting 212 UNSEGMENTED WORMS to notice that the Turbellarians and TemnocephaUds are free-Uving, except in the case of a few marine Turbellarians which have taken to parasitism ; that the Trematodes are all parasitic, either external hangers-on (ectoparasites) or internal boarders (endoparasites) ; and that the Cestodes are altogether endoparasitic. It is probable that the flukes and tape-worms arose from Turbellarian-like ancestors which adopted parasitic habits. Attention must be directed to the flame-cells which are characteristic of Platyhel- minthes. Each terminal twig of a branch of an excretory canal leads into a large hollow cell, from the base of which a bunch of cilia — with rapid movements suggesting a flickering flame — projects into the cavity towards the lumen of the twig. Class TuRBELLARiA. Planarians, etc. Turbellarians are tinsegmented " worms'' usually leaf- like, living in fresh, brackish, or salt water, or in moist earth. Almost all are carnivorous, a few are parasitic. They represent the beginning of definite bilateral symmetry. The ectoderm is ciliated, often glandular, often with peculiar rod-like bodies (rhabdites) which may be discharged on irrita- tion. A pair of ganglia in the anterior region give off lateral nerve-cords, and there are usually simple sense organs. The food canal has a protrusible muscular pharynx, is often branched, and is always blind ; digestion takes place partly or wholly within the lining cells. There are no special respiratory or circulatory organs ; the body cavity is not represented, unless it be by intercellular lacunce in the parenchyma ; the excretory system usually consists of two longitudinal canals, whose branches end internally in flame- cells. The Turbellarians are almost always hermaphrodite ; and the reproductive organs usually show some division of labour, e.g. in the development of a yolk gland, which may have arisen as an over-nourished {hypertrophied) part of the ovary. The eggs are usually enclosed in shells or cocoons, and the development may include a metamorphosis. Some forms multiply by fission. There seem to be affinities between Turbellaria and Coelentera, especially the CtenoPhora. CHARACTERS OF TURBELLARIA 213 The Turbellarian worms form an exceedingly interesting group ; they are often beautiful, and the ciliated ectoderm and well-developed muscles enable them to move with singular grace. Although the bilateral symmetry and the distinction of anterior and posterior ends is quite marked, the " mouth " or single opening of the food canal is often near the middle of the ventral surface. The anterior region is usually furnished with tactile processes. The shape of the body in the aquatic Fig. 115. — Diagram of Turbellarian. — After Lang. C, Cerebral ganglia ; E., eye ; T., tentacle ; PH., pharynx ; Mo., mouth ; M., male aperture ; P., female aperture ; the ovaries and testes are branched organs on both sides, represented b^ dots. forms is flattened and leaf-like, as in the delicate Leptoplana, the " living film " found on the shore-rocks. Fresh-water forms are usually small and often minute, but those living in the sea may attain a length of six inches, though most are small. Land Planarians are elongated and more worm-like in shape ; they may measure a foot or more in length, and are most abundant in tropi-zal countries. Some, like Planaria, have so much regenerative capacity that half a dozen or more may be produced by cutting one into pieces. 214 UNSEGMENTED WORMS Classification. — Order i. Rhabdocoelida — small fresh-water and marine forms. The food canal is very slightly branched, or quite straight, or blocked. Rhabdocoela. With straight intestine, e.g. Microstoma, a fresh- water genus. It is first male and then female (protandrous hermaphrodite) ; it forms temporarily united asexual chains, sometimes of sixteen individuals, suggesting the origin of a segmented type. Grafilla and Anoplodium are parasitic on Gastropods. Among the Vorticidae allied to Grafilla we may notice Provortex tellincB in Tellina and a related form in the cockle. Alloiocoela. With irregular caeca on the gut, e.g. Allostoma. All marine except one from Swiss lakes {Plagiostoma lemani) and Bothrioplana. Accela. Without intestine, e.g. Convoluta, which contains green symbiotic alg«. Marine. Order 2. Tricladida. Elongated flat " Planarians " with three main branches from the gut, e.g. Planaria and Dendroccelum (fresh-water), the former sometimes dividing transversely ; Polycelis nigra, a common fresh-water form ; Gunda (Procerodes) segmentata (marine), showing hints of internal segmentation ; Geodesmus and Bipalium (in damp earth) ; Bipalium kewense is an import often found in Britain. Order 3. Polycladida. Large leaf-like marine " Planarians," with numerous intestinal branches diverging from a central stomach, e.g. Leptoplana (not uncommon on the seashore), Thysanozoon. Class Temnocephaloidea The Temnocephalids are flattened forms, e.g. Temnocephala, found clinging to fresh-water animals, especially Crustaceans ; there is a large ventral sucker ; the epidermis is a nucleated syncytium (i.e. without distinct demarcation into cells) which secretes a thick cuticle, contains rhabdites, and rarely bears cilia. The class seems to be intermediate between Rhab- docoelid Turbellaria and Trematodes. Symbiotic algse. — Of all the numerous Invertebrates which harbour symbiotic algae within their bodies the best studied is the Acoelan Convoluta, thanks especially to Keeble and Gamble. In C. roscoffensis the algae are green {Zoochlorellce) ; like other green plants, they utilise the energy of sunlight to build up complex organic com- pounds from carbon dioxide, with evolution of oxygen. Both the oxygen and the elaborated compounds (food- stuffs) are valuable to the host ; for the greater part of its life C roscoffensis does not feed for itself, but lives on the SYMBIOSIS 215 products of its symbionts. In C. paradoxa the algae are yellow {Zooxanthellce), but their function is the same ; this species does seek food on its own account, but it cannot live without the help of the algae. Zoochlorellae occur in many Rhizopods and Ciliates, in fresh-water Sponges, in Chlorohydra, and in some Rotifers ; Zooxanthellae occur in many Rhizopods and most Radiolarians, and in very many Ccelenterates, especially Anthozoa. In many cases the host can probably survive without the symbionts, but they undoubtedly help it, especially in times of starva- tion. Sometimes, in unfavourable circumstances, the host will kill the goose that lays the golden eggs by digesting the algae. In return for their services to the host, the algae make use of the carbon dioxide and nitrogenous waste products of the host's metabolism. Class Trematoda. Flukes, etc. The Trematodes are leaf-like^ or sometimes cylindrical external or internal parasites. With their parasitic life may be associated the absence of cilia on the surface of the adults, the thick " cuticle'' the presence of attaching suckers {occasion- ally with hooks), and the rarity of sense organs. After embryonic life the ectoderm degenerates, ceases to be distinctly cellular, and sinks inwards. It is likely that they have arisen from free Turbellarian-like ancestors, and they resemble the Turbellarians in being unsegmented, in having anterior ganglia, from which nerves pass backward and forward, in the rudimentary nature of the body cavity, in the ramifying system of fine excretory canals, in the hermaphrodite and usually complex reproductive system. The excretory and nervous systems are, however, more complex than those 0^ Turbellaria. The alimentary canal is usually forked, often much branched, and always ends blindly. In many cases the animals are self -impregnating, but cross- fertilisation also occurs. The development of the external parasites is usually direct, of the internal parasites usually indirect, involving alternation of generations. They occur on or in all sorts of Vertebrates, but those which have an indirect development, and require two hosts to complete their life-cycle, often pass part of their life in some Invertebrate. 2l6 UNSEGMENTED WORMS Type The Liver Fluke {Distomum hepaticutn) The adult fluke lives as a parasite in the liver and bile ducts of the sheep, causing liver-rot." flukes, it occasional sometimes e.v FiG.i 1 6. —Structure of liver fluke. — After Sommer. From ventral surface branched gut (g.) and the lateral nerve [l.n.) are shown to the left, the branches of the excretory vessel {e.v.) to the right. m.. Mouth ; ph., pharynx ; g., lateral head ganglion ; v.s., ventral sucker ; c.s., position of cirrus sac. An arrow indicates the ex- cretory aperture. Unlike most has many hosts — it occurs in cattle, horses. deer, camel, antelopes, goat, pig, beaver, squirrel, kangaroo, and rarely in man. The animal is flat, oval, and leaf-like, almost an inch in length by half an inch across the broadest part, reddish brown to greyish yellow in colour. As the word Distomum suggests, there are two suckers — an an- terior, perforated by the mouth ; a second, im- perforate, a little farther back on the mid-ventral line. There is a muscular pharynx and a blind alimentary canal which sends branches through- out the body. The food is the blood sucked from the liver of the host. From a ganglionated The collar round the pharynx, nerves go forward and backward ; of those which run backward, the two lateral are most im- portant. Although the larva has eye spots to STRUCTURE OF LIVER FLUKE 217 Start with, there are no sense organs in the adult. The body cavity is not represented unless it be by minute P- c.s — ^ -<^' t^ ^^^-^ Fig. 117. — Reproductive organs of liver fluke.— After Sommer. ov. Ovary (dark). ut. Uterus. c.s. Cirrus sac. /. Female aperture. s.v. Seminal vesicle. y.gl. Diffuse yolk glands. sh.g. Shell gland. v.d. Vas deferens. T. Testes (anterior). p. Penis. m. Mouth. g. Anterior lobes of gut. intercellular spaces in the body parenchyma. Into these there open the internal ciliated ends of much-branched excretory tubes (see Figs. 114 and ii6), which unite 21 8 UNSEGMENTED WORMS posteriorly in a terminal vesicle opening to the exterior. The reproductive system is hermaphrodite and complex. From much-branched testes, spermatozoa pass by a pair of ducts (vasa deferentia) into a seminal vesicle lying in front of the ventral sucker. Thence they are expelled by an ejaculatory duct, which passes through a muscular protrusible penis. The retracted penis and the seminal vesicle lie in a space or " cirrus sac " between the ventral sucker and the external male genital aperture. The ovary is also branched, but less so than the testes. The ova pass from its tubes into an ovarian duct. Nutritive cells are gathered from very diffuse yolk glands, collected in a reservoir, and pass by a duct into the end of the afore- said ovarian duct. At the junction of the yolk duct and the ovarian duct there is a shell gland, which secretes the " horny " shells of the eggs, and from near the junction a fine canal (the Laurer-Stieda canal) seems to pass direct to the exterior, opening on the dorsal surface. The meaning of this is still somewhat uncertain. In some flukes it is said to be a copulatory duct ; in others it is regarded as a safety valve for overflowing products. From the junction of the ovarian duct and the duct from the yolk reservoir, the eggs (now furnished with yolk cells, accompanied by spermatozoa, and encased in shells) pass into a wide convoluted median tube, the oviduct or uterus, which opens to the exterior at the base of the penis. Self-fertilisation is probably normal, but in some related forms cross-fertilisation has been observed. Life-history. — The fertilised and segmented eggs pass in large numbers from the bile duct of the sheep to the intestine, and thence to the exterior. A single fluke may produce about 50,000 embryos, which illustrates the prolific reproduction often associated with the luxurious conditions of parasitism, and almost essential to the con- tinuance of species whose life-cycles are full of risks. Outside of the host, but still within the egg-case, the embryo develops for a few weeks, and eventually escapes at one end of the shell. Those which are not deposited in or beside pools of water soon die. The free embryo, known as a miracidium, is conical in form, covered with cilia, provided with two eye-spots, and actively locomotor. By means of its cilia it swims actively in the water for some hours, but its sole chance of life depends on its meeting a small amphibious water-snail (Limnceiis truncatulus or minutus), into which it bores. In an epidemic among horses and cattle in the Hawaiian Islands, the host was L. oahuensis ; in the same locality the host may be DEVELOPMENT OF DISTOMUM 219 Fig. 118. — Life-history of liver fluke. — After Thomas. I, Developing embryo in egg-case ; 2, free-swimming ciliated larva ; 3, sporocyst ; 3a, shell of LimncBus truncahdus ; 4, division of sporo- cyst ; 5, sporocyst with redis forming within it ; 6, redia with mor rediae forming within it ; 7, tailed cercaria ; 8, young fluke. 220 UNSEGMENTED WORMS L. peregra ; in Victoria Bulimus tenuistriatus . This diversity of host, also remarkable in the adult, is very unusual. Within the snail, e.g. in the pulmonary chamber, the miracidium settles down, loses its cilia, increases in size, and becomes a sporocyst. The sporocyst is a hollow sac, with a slightly muscular wall and with the beginnings of an excretory system. Sometimes this sporocyst divides transversely (Fig. ii8 (4)). Within the sporocyst a few cells behave like partheno- genetic ova. Each segments into a ball of cells or morula, which is invaginated into a gastrula, and grows into another form of larva — the redia. These rediae burst out of the sporocyst, and migrate into the liver or some other organ. Each sporocyst usually forms at a time 5-8 rediae ; each of these forms 8-12 more rediae ; and each of these forms 14-20 cercariae. In the winter a sporocyst may give rise to cercariae directly. A redia is a cylindrical organism with a short alimentary canal, excretory canals with " flame cells," and a pair of blunt locomotor processes posteriorly. A cercaria has a bifurcated gut, two suckers, a locomotor tail, and the beginnings of gonads (Fig. 118 (6)). The cercariae emerge from the rediae, wriggle out of the snail, pass into the water, and after swimming for a short time, moor themselves to stems of damp grass. There they lose their tails and become encysted. If the encysted cercaria on the grass stem be eaten by a sheep, the cyst is dissolved in the stomach, and the young fluke makes its way up the bile duct and its tributaries. In about six weeks it grows into the adult sexual fluke. It will be noted that the sporocyst is the modified embryo, but that it has the power of giving rise asexually to redia?. These develop, however, from special cells of the sporocyst, which we may compare to spores or to precociously developed parthenogenetic ova. Though the reproduction is asexual, it is not comparable to budding or division. The same power is possessed by the rediae, and there are thus several (at least two) asexual generations between the embryo and the adult. The disease of liver -rot in sheep is common and disastrous. It has been known to destroy a million sheep in one year in Britain alone. Classification. — Order i. Heterocotylea, with a posterior ad- hesive organ, often with a pair of accessory suckers beside the mouth. LIFE-HISTORY OF LIVER FLUKE 221 RtDlAE 5POROCY5T Fig. 119. — Diagram of life-cycle of liver fluke. Upper quadrant, adult in sheep ; />/;., pharynx ; s., sucker; g., gut. Right quadrant, free-swimming larva with eye-spots {e.). Lower quadrant, sporocyst and rediae in water-snail ; R., redia within sporocyst or within redia ; g., gut in redia ; C, cercariae in redia. Left quadrant, free cercaria ; g., gut ; s., sucker ; t., tail. 222 UNSEGMENTED WORMS Most are ectoparasitic. The development is direct and associated with one host (monogenetic). e.g. Polystonium integerrimum, with many posterior suckers, often in the bladder of the frog. It attaches itself in its youth to the gills of tadpoles, passes thence through the food canal to the bladder. Gyrodactylus, on the gills and fins of fresh-water fishes. It is Fig. 1 20. — Male and female Bilharzia — Schistosomum hcemato- bium. — After Looss. The male is about three- fifths of an inch long ; the female (F.), carried in the ventral groove or gynaeco- phoric canal {G.C.), is four- fifths. S., Anterior suctorial mouth ; P.S., the adhesive sucker. On the surface of the male's body there are numerous minute papillae. viviparous, but the embryo, before it is extruded, itself contains an embryo, and this in turn another. Diplozoon paradoxum consists of two individuals united. The single larva (Diporpa) is at first free-swimming, but becomes a parasite on the gills of a minnow, and there two individuals unite very closely and per manently. Tristomum, with three suckers, on some marine fishes. ORDERS OF TREMATODA 223 Order 2. Aspidocotylea, with a large sucker occupying most of the ventral surface. Development is direct, and there is one host. e.g. Aspidogasier in Molluscs. Order 3. Malacotylea, with never more than two suckers. The development is indirect and requires two hosts, the adult usually frequenting the gut of a vertebrate. e.g. Disiomum, with numerous species. Schistosomum (Bilharzia) hcematohium, a parasite of man, widely distributed in Africa, e.g. in Egypt. It occurs in the portal vein, the blood vessels of the bladder, etc., 'causing inflammation, ha;maturia, stone, etc. The embryos are passed out in the urine. The intermediate host is a fresh-water snail {e.g. Biilinus). There is no redia. The bifid microscopic Cercaria usually enters man by the skin. The pain is due to the sharp corners of the egg-shells, which have terminal spines. Another species, S. mansoni, is intestinal, and the eggs, which have a lateral spine, pass out with the faeces. The young stages occur in Planorbis, etc. The cercariae die in 36 hours in water kept quite still, or may be killed by a little sulphate of soda. Monostomum, with one sucker ; adult in ducks, young in fresh-water snail, Planorbis. The relationships of the Trematodes are on one hand with the free-living Turbellarians, on the other hand with the parasitic Cestodes. Class Cestoda. Tape-worms The Cestodes are internal parasites., whose life-history includes a bladder-worm (proscolex) and a tape-worm (strobila) stage, the former in a Vertebrate or Invertebrate host, the latter {with one exception) in a Vertebrate. In a few cases the body is unsegmented, e.g. Archigetes and Caryophyllaeus, with one set of gonads ; in a few others, e.g. Ligula, there is a serial repetition of gonads without distinct segmentation of the body ; in most cases, e.g. Taenia and Bothriocephalus, the body of the tape-worm forms a chain of numerous joints or proglottides, each with a set of gonads. Thus the class in- cludes transitions from unsegmented to segmented forms, but the latter are imperfectly integrated? The general form of the body is tape-like and bilaterally symmetrical, with anterior hooks, grooves, or suckers ensuring attachment to the gut of the host. The body wall consists of a cuticle and a well-innervated epidermis, within which there is parenchymatous connective tissue, often with cortical deposits of lime, and at least two sets {longitudinal and transverse) of unstriped muscles. The nervous system consists of two or more longitudinal nerve- 224 UNSEGMENTED WORMS Strands and anterior commissures ; there are no special sense organs. There is no alimentary system ; the parasite floating in the digested food of its host absorbs soluble material by its general surface. There is no vascular nor respiratory system, and a body cavity is represented merely by irregular spaces in the solid parenchymatous tissue. In some of these spaces there are '' flame- cells,'' which lie at the ends of the fine branches of lofigitudinal excretory tubes, which are united in a ring in the head, are connected transversely at each joint, and open terminally by one or more pores. All tape-worms are hermaphrodite, and most, if not all, are probably self -fertilising . The male reproductive organs in- clude diffuse testes, a vas deferens, and a protrusible terminal cirrus. The female organs include a pair of ovaries, yolk glands, a shell gland, a vagina by which spermatozoa enter, a receptacle for storing spermatozoa, and a uterus in which the ova develop. The embryo develops within another host into a proscolex or bladder-worm stage, which forms a " head " or scolex. When the host of the bladder-worm is eaten by the final host, the scolex develops into an adult sexual tape-worm. With the conditions of endoparasitic life may be associated the occurrence of fixing orgafis, the absence of sense organs, the low though somewhat complex character of the nervous system, the entire absence of a food canal, and the prolific reproduction. Life-history of Taenia solium. — This is one of the most frequent of the tape- worms infesting man. In its adult state it is often many feet in length, and is attached by its " head " to the wall of the intestine. The head bears four suckers and a crown of hooks, and buds off a long chain of joints, which develop complex reproductive organs as they get shunted farther and farther from the head. The last of the joints or proglottides is liberated (singly or along with others), and passes down the intestine of its host to the exterior. It has some power of muscular contraction and of movement, and it is distended with little embryos within firm egg-shells. When the proglottis ruptures, these are set free. In certain circumstances, the embryos, within their firmly resistant egg-shells, may be swallowed by the omni- vorous pig. Within its alimentary canal the egg-shells are dissolved, and embryos (hexacanths) bearing six anterior TAPE-WORMS 225 hooks are liberated. They bore their way from the in- testine into the muscles or other structures, and there encyst. They lose their hooks, increase in size, and become passive, vegetative, asexual " bladder-worms." A bud from the wall of the bladder or proscolex grows into the cavity of the same, and forms the future " head " or Fig. 121. Front end of the head of Tcenia solium, showing four adhesive suckers and a circle of fixing hooks. The natural size is that of an ordinary pin's head. scolex. This is afterwards everted, ajid then the bladder- worm consists of a small head attached by a short neck to a relatively large bladder. When man unwittingly eats " measly " pork — that is, pork infested with bladder-worms — an opportunity for further development is afforded. The bladder is lost, and is of no importance, but the '* head " or scolex fixes itself to the wall of the intestine. There it is copiously and 15 226 UNSEGMENTED WORMS richly nourished, and buds off asexually a chain of joints. As these joints are pushed by younger interpolated buds 7'.S Fig. 122.- -Diagram of reproductive organs in Cestode joint. Constructed from Leuckart. ov., Ovary, with short oviduct ; «/., " uterus " ; /., diffuse testes ; sh.g., shell glared; y.g., yolk gland; v.d., vas deferens; v., vagina ; r.s., receptaculum seminis ; I.e., longitudinal excretory ducts ; I.e., transverse bridges connecting these. The dotted lines above and below represent the anterior and posterior borders of the proglottis. Note that the so-called uterus is blind ; it opens to the exterior in a few tape-worms, and is perhaps the homologue of the Laurer-Stieda canal of Trematodes. farther and farther from the head, they become sexually mature. The ova are fertilised, apparently by spermatozoa LIFE-CYCLE OF TAPE-WORM 227 from the same joint ; the joint becomes distended with developmg embryos. These ripe joints are hberated, the Fig. 123— Life-history of Tcznia solium.— Aitev Leuckart '' S'^,h^°'^'^d embryo in egg-case ; 2, proscolex or bladder-worm atef 'hrad T^T'^^^'^^^J 3, bladder-worm wit?"g™ embryos are set free by rupture, and the vicious circle may recommence. Happily, however, the chances are 228 UNSEGMENTED WORMS many millions to one against the embryo becoming an adult. The above history is true, mutatis mutandis, for many other tape- worms. The embryo grows into a proscolex or bladder, which buds off a scolex or head, which, in another host, buds off the chain of proglottides. HEAP Fig. 124. — Diagram of Ufe-history of TcBnia solium. First chapter : Tape-worm in man ; H., head ; PR., proglottides. Second chapter : Free proglottis and egg-cases ; ut., uterus ; g.a., genital aperture ; embryo within the egg-case. Third chapter : Within the intermediate host, the pig ; H., hexacanth embryo ; p.sc, proscolex or bladder-worm ; m., muscle of pig ; sc, scolex or head, everted in final host. As it is virtually the same animal throughout, the life-history does not include an " alternation of generations." It is doubtful, however, what term should be applied to those cases in which the bladder-worm (Ccenurus and Echinococcus) forms not one head only but many, each of which is capable of becoming an adult tape-worm. The only known eTtception to the fact that sexual tape-worms are parasites of Vertebrates is Archigetes sieboldii, a simple cestode which is sexual within the small fresh-water oligochaete Tubifex rivulorum. TAPE-WORMS 229 Representative Life-Histories Adult, Sexual, or Tape-worm Stage. 1. Tcenia solium, in man, with four suckers and many hooks. The joints are elongated ; the ripe uterus shows coarse branching. 2. TcBnia saginata, in man, with four suckers, but no hooks. The joints are markedly elongated ; the ripe uterus has many slender branches. 3. Bothriocephalus latus (Dibothrio- cephalus), in man, with two lateral groove-like suckers, but no hooks, with less distinct separation of the pro- glottides than in Tcenia. The joints, which are short and wide, show less distinct separation. The ripe uterus is somewhat stellate. The total length of the chain may be as much as 11 yards. Common in Finland and Switzer- land. 4. Echinococcifer echinococcus, in dog, wolf, jackal. Very small, with three joints behind the head, which bears four suckers and two rows of barbed booklets. 5. Tcenia ccenurus, in dog. 6. Tcenia serrata, in dog. 7. Dipylidium caniniim (T. cucumer- ina), in cat and dog ; head with hooks and four suckers ; joints ovoid, with genital aperture at both margins. 8. Moniezia, the broad tape-worm of sheep and cattle. Non-Sexual, Proscolex, or Bladder- worm Stage. 1. Cysticercus cellulosce, in muscles of the pig. 2. Bladder-worm in cattle. 3. The ciliated, free-swimming em- bryo becomes a parasite in the muscles of pike, trout, burbot, etc., but without a distinct bladder-like stage. It is worm-like in appearance, and called a plerocercoid larva. 4. Echinococcus veterinonim, in sheep, cattle, pigs, etc., and sometimes in man, producing brood capsules, which give rise to many " heads." 5. Cosmirus cerebralis, causing sturdie or staggers in sheep, with numerous " heads." Also in cattle, goat, horse, etc. 6. Cysticercus pisiformis, in rabbit. 7. In lice and fleas. 8. Life-history unknown. Zoologically the cestodes are interesting, on account of their life- histories, the degeneration associated with their parasitism, the pre- valence of self-impregnation, and the complexity of the reproductive organs. Practically they are of importance as parasites of man and domestic animals. Classification. — The class Cestoda includes a number of families : — Cestodariidae. No joints, one set of gonads. e.g. Archigetes, Caryophyllceus, Amphilina, Gyrocotyle, Bothriocephalidae. Two weak fiat suckers ; genital openings usually on the flat surfaces. e.g. Bothriocephalus ; Ligula, with no suckers or joints but with serial gonads. 230 UNSEGMENTED WORMS Tetrarhynchidse. With four protrusible proboscides armed with hooks, parasites of fishes. Found also in Sepia. e.g. Tetrarhynchtis. The finest pearls in the Ceylon pearl oyster are formed round a larval Tetrarhynchus. Tetraphylhdae. With four very mobile suckers. e.g. Echeneibothrium, Phyllo- bothrium. Taeniidae. With four suckers, often with apical hooks, with margi- -j^ nal genital apertures. e.g. Tcsnia. General Note on Platyhelminthes The four classes, Turbellaria, Trematoda, Cestoda, and Temno- cephaloidea, constitute the Platy- helminthes or Flat -worms — an in- teresting group, because its members illustrate so well the progressive degeneration associated with increas- ing parasitism, and also because of the relatively great simplicity. The four classes are nearly related, for forms like Temnocephala connect Turbellaria and Trematoda, and the " monozoic " Cestodes like Archi- getes, Amphilina, CaryophyllcBus, and Gyrocotyle connect Trematoda and Cestoda. It is probable that both Cestodes and Trematodes arose from a Turbellarian stock. Among the most striking of the Platyhelminth characters are the nature of the excretory and repro- ductive organs and the condition of the mesoderm. The excretory system, with its longitudinal trunks, its ramifying canals, and " fiame-cells," is characteristic. The reproductive Fig. 125. — Diagrams of bladder-worms. I. The ordinary Cysticercus type, with one head (H.). II. The Coenurus type, with many heads. III. The Echinococcus type, with , , j- ■ • many heads, and with brood organs are complex, show division capsules producing many of labour, and are furnished with ^^^^s- ducts of their own, unconnected with the excretory system — a condition not common in worms. The presence of shells around the eggs is another point of interest. It becomes of great importance to the parasitic flukes and tape-worms, but occurs also in the free-living Turbellaria. The formation of yolk cells from a specialised part of the ovary (yolk gland) is also noteworthy. There is no true body cavity, RIBBON-WORMS 231 —po the space between gut and body wall being filled with a packing tissue ; the absence of an anus is also important, the two characters taken together being held to indicate affinity with the Ctenophora. Class Nemertinea. Phylum Nemertea. The ribbon-worms or Nemertincs are interesting in many ways, e.g. in being the simplest animals ^^^^^^^—pp to have an open gut, a closed blood- system, and, occasionally, haemoglobin ; in having some very peculiar structures, notably a protrusible proboscis and ciliated head slits ; in being in many cases extraordinarily extensile and liable to break into pieces. The Nemertines are worm-like animals, unsegmented and generally elongate in form ■ they are almost all marine, and most, if not all, are carnivorous. The ectoderm is ciliated. There is a remarkable retractile proboscis, uncon- nected with the alimentary canal, and forming a tactile organ or a weapon. The nervous system consists of a brain, a com- missure round the proboscis, and two lateral nerve-cords ; in connection with the brain there is a pair of ciliated pits. The gut terminates in a posterior anus, and is furnished with lateral pockets. ' There is no body cavity in the adult, but the closed vascular system is probably of coelomic origin. The excretory system is apparently of the Platyhelminth type. The sexes are usually separate and the si -^ Fig. 126. — Diagrammatic longitudinal section of a Nemertine (Amphiporus ladifloreiis), dorsal view. — After M'Intosh. p.p., Proboscis pore ; b., brain giving off the latera^ nerve-cords (n.) ; po., oesophageal pocket ; p., pro- boscis Iving within its sheath ; St., stilet of proboscis ; m., retractor muscles of proboscis ; g., gut shown in outline at the sides of the proboscis ; e., the three main longitudinal blood vessels, which unite both anteriorly and posteriorly. 232 UNSEGMENTED WORMS organs simple. The development is in some cases direct, while in others there is a peculiar pelagic larva. General Account of Nemertines In appearance most Nemertines are ribbon- or thread-like, and the cross-section is generally a flattened cylinder. They show a greater diversity of size than any other " worms " — from a Linens, 12 or more d.n P Fig. 127. — Transverse section of the Nemertine Drepanophorus latus. — After Biirger. d.n., Dorsal or proboscis nerve ; P.s., proboscis sheath ; P.c, proboscis cavity ; P.s'., sac of proboscis cavity ; d.v.m., dorso-ventral muscles ; cm., circular muscles ; l.m., longitudinal muscles ; l.n., lateral nerve with branches ; P., parenchyma ; g., gut ; l.v., lateral blood vessel, beside which lies an excretory vessel ; E.p., excretory pore ; d.v'., dorsal blood vessel ; Ep., epidermis. feet in length (25 metres has been recorded for an extended Lineus longissitnus), to the pelagic Pelagonemertes, which is under an inch. The colours are often bright, and tend to resemble those of the sur- roundings. The ectoderm is covered with numerous short cilia, and many of its cells are also glandular, secreting the mucus, which often forms a tube around the animal, or is exuded in movement. Beneath the epidermis there is a parenchyma, consisting in part of connective tissue, and often in part gelatinous. The body is remarkably con- tractile, and in some cases the spasms result in breakage. The muscles are circular and longitudinal, and often also diagonal. The fibres are striped. In the adult there is no distinct coelom, the space between RIBBON-WORMS 233 the gut and the body wall being filled up with gelatinous connective tissue. In the larva?, however, a body cavity may be seen, either as an archiccfile, i.e. the persistent segmentation cavity {IJneus obscurus), or as a schizocoele, i.e. a space formed by the cleavage of the mesoderm into two layers (Pilidium-larvse). In the adult only the blood spaces and the cavity of the proboscis sheath are coelomic. The nervous system" consists of a brain generally four-lobed — the two lobes of each side being closely united and connected with those on the other side by a commissure above and by another below the proboscis cavity. From the lower lobes two longitudinal nerve-stems run along the sides, and are sometimes united posteriorly above the anus (Fig. 126, n.). In some forms there is in addition a dorso-median nerve, and sometimes a ventro-median nerve. On each side of the head there is a ciliated pit communicating with the exterior through an open slit or groove, and communi- cating internally either with the brain itself or with ad- jacent nervous tissue. In those cases in which the de- velopment has been studied, these so-called lateral organs arise from ectodermic insink- ings and oesophageal out- growths. In the most primi- tive genus, Carinella, they are absent, except in one species. It has been sug- gested that they conduce to the respiration of the brain, which is rich in haemoglobin, and they have even been compared with gill-slits. In some forms the groove through which they open to the exterior is rhythmically gested that they are sensory are very sensitive; ficial nerve plexus. eyes and eye spots are general ; and in some there are otocyst-sacs. Apart from the cephalic $hts, the head also bears sensory pits and grooves and terminal sensory spots. In some there is a pair of lateral sense organs in the (anterior) excretory region. The mouth is ventral, and leads into a plaited glandular fore-gut or cBsophagus, which is followed by a straight, ciliated mid-gut (stomach and intestine), usually with regularly arranged lateral casca. Between the caeca run transverse muscle partitions. The anus is in most cases terminal. In a cavity along the dorsal median line there hes the remarkable proboscis. It is protruded and retracted through an opening above, or, in a few cases, within the mouth. It arises in the body wall and is surrounded by a cavity (rhynchocoelom) bounded by Fig. 128. — Transverse section of a simple Nemertine {Carinella). — After Biirger. d.n., Dorsal nerve ; p.c, proboscis cavity ; g., gut ; cm., circular muscles ; l.m., longitudinal muscles ; d.v.m., dorso-ventral or diagonal muscles ; l.v., lateral blood vessel. contractile. It has also been sug- Apart from these organs, Nemertines and in many this is associated with a super- Tactile papillae and patches are often present ; 234 UNSEGMENTED WORMS a muscular proboscis sheath. The proboscis is a muscular, richly innervated tube hned with glandular epithelium, sometimes protruded with such force that it separates from the body. It has been compared in its retracted state to a glove-finger drawn in by two threads attached to its tip, the threads being retractor muscles which are fastened posteriorly to the wall of the proboscis sheath. But in front of the attachment of the retractor muscles there is a non-eversible glandular region which secretes an irritant fluid. In many cases there are stilets at the tip of the eversible portion, and if these be absent, there are adhesive papillae. There is a hint of a similar structure in some Rhabdoctjel Turbellarians, and the organ may be interpreted as origin- ally tactile, secondarily aggressive. It is protruded by the muscular contraction of the walls of the proboscis sheath, which forms a closed cavity surrounding the proboscis, and containing a fluid with corpuscles (Fig. 126). In the majority there are three longitudinal blood vessels or spaces, a median and two laterals, which unite anteriorly and posteriorly, and also communicate by numerous transverse branches. The vessels or spaces are remnants of a ccelom. The blood is a coloiirless fluid, sometimes at least with nucleated elliptical corpuscles in which haemo- globin may be present. The excretory system usually consists of two coiled ciliated canals opening in the anterior region by a varying number of ducts. They are said to divide up internally into numerous fine branches ending in flame- cells, or in blind ampuUee embedded in the walls of the blood vessels. The sexes are usually separate, and the reproductive organs are always simple. A few species (of Geonemertes and Prosadenophorus) are hermaphrodite, and some species of Tetrastemma are protandrous. The organs consist of simple sacs, arranged in a series on each side between the intestinal cseca, and communicating with the exterior by fine pores. The ova are often laid in gelatinous tubes, and are probably fertilised shortly before or at the time of expulsion. In three or four forms {Prosorhochmus, a fresh- water Tetrastemma, a species of Linens) known to be viviparous, the fertilisation must, of course, be internal. Segmentation is total and almost always equal ; a complete or partial gastrula is formed, and development may be direct or indirect. In Cerebratulus, etc., the larva is adapted for pelagic life, and is known as the Pilidium. " In external shape it resembles a helmet with spike and ear lobes, the spike being a strong and long flagellum or a tuft of long ciha, the ear lobes lateral ciliated appendages " (Hubrecht). Out of this, somewhat abruptly, the adult form arises. Relationships. — The Nemertines are probably nearly related to Turbellaria, but show some very distinct marks of advance. Of these, the most noticeable are the presence of an anus, of a closed vascular system, of a coelom at least in the larva. The presence of flame-cells in connection with the excretory system confirms the idea of Platy- helminth affinities ; but it is to be noticed that the reproductive system is strikingly different. Professor Hubrecht has suggested that Nemertines exhibit affinities with Vertebrates, comparing proboscis sheath with notochord, and so forth. RIBBON-WORMS 235 Classification. Order Protonemertini. Brain and lateral nerves outside the muscular layers ; mouth behind brain ; no stilets. Carinella, Hubrechtia. Order Mesonemertini. Lateral nerves in the muscular layer ; mouth behind brain ; no stilets. Carinoma, Cephalothrix. Order Metanemertini. Mouth in front of brain, usually opening along with proboscis ; usually with stilets ; lateral nerves internal to the muscular layers ; usually with an intestinal caecum. e.g. Amphiporus, Drepanophorus, Tetrastemma. An isolated form, Malacobdella, parasitic in bivalves, has a posterior sucker, a coiled intestine, and other peculiarities. Order Heteronemertini. Mouth behind brain ; no stilets ; three layers of muscle, the outermost and innermost longitudinal ; lateral nerves outside circular muscular layer. e.g. Lineus, Cerebratulus.^ Habits. — Most Nemertines are marine, creeping about in the mud, under stones, among seaweed, and the hke ; many, e.g. Cerebratulus, are able to swim ; Pelagonemertes and Planktonemertes are leaf-Hke hyaline forms of pelagic habit ; two or three species of Prostoma live in fresh water ; seven species of Geonemertes are terrestrial ; Malacobdella and a few others live in the mantle-cavity of marine bivalves, and some others are found as commensals in Ascidians ; Cephalothrix galathece destroys the eggs of its host — the crustacean Galathea. Most seem to be carnivorous, eating annelids, molluscs, and even small crustaceans. Many break readily into pieces when irri- tated, and some are able to regenerate what they lose in this way. The fresh-water Prostoma lumbricoides forms a pro- tective cyst of mucous threads in unfavourable conditions, and Tetrastemma dorsale often does the same along stems of the hydroid Tubularia. 236 unsegmented worms Phylum Nematohelminthes Class Nematoda, e.g. Ascaridae. Class Nematomorpha, Gordiidae. Class Acanthocephala, e.g. Echinorhynchus Class Nematoda. Thread-worms, Hair-worms, etc. The Nematodes are unsegmented, more or less thread-like " worms,"" some free-living and others parasitic. The body is covered by a cuticle, often thick, usually striate, often subject to moulting ; the muscular system consists of elongated muscle- cells arranged longitudinally, and usually leaving two free " lateral lilies." From a nerve-ring around the gullet, six or so nerves go forwards and also backwards. The gut is usually well developed, with mouth and anus, and is divided into three regions. Vascular and respiratory systems are unrepresented ; the cavity of the body is not coelomic ; the remarkable excretory system consists of two lateral canals opening anteriorly by a single pore. The sexes are usually separate and the reproductive organs simple ; there is distinct sexual dimorphism . The males have usually copulatory spicules, and sometimes a membranous bursa. The vulva may be any- where on the ventral surface, often well forward. The life-history is often intricate. There are many remarkable features such as the sluggish amoeboid spermatozoa, the almost complete absence of cilia and flagella, and the absence of migratory phagocytes. Type, Ascaris megalocephala, the Round- worm of the horse This round-worm occurs in the small intestine of the horse, while other species similarly infest man, ox, pig, etc. The body is cylindrical in cross-section and tapering at each end. The colour is dead-white, the absence of pigment being very characteristic of Nematodes. Some of the small thread-worms, e.g. Trichostrongylus pergracilis in the caeca of the grouse, are quite transparent- and almost invisible when alive. At the anterior end is the mouth, furnished with three lips bearing sense papillae ; the anus is posterior and ventral. The male is smaller than the female, and has a recurved tail furnished with two horny spines and numerous sense papillae. It is usually about seven inches long, while the female may be as much as seventeen. ROUND-WORMS 237 {a) Most externally there is a thick chitinoid cuticle, perhaps of protective value. With its presence may be associated the scarcity of cutaneous glands, and the absence of cilia, (b) Beneath this is the sub-cuticula or epidermis, thickened along four longitudinal lines — median, dorsal, ventral, and lateral — and consisting of a protoplasmic matrix without distinct cell-limits. Except at the tail-end the nuclei are confined to the longitudinal lines, and are most Tl.C ovo Fig. 129. — Cross-section through Ascaris. — From a speoimen. DN., Dorsal nerve; n.c, non-contractile portion of muscle cells; C, cuticle: £., epidermis; LL., lateral line; EV., excretory vessel ; A/.', contractile portion of muscle cells ; VN., ventral nerve ; OV., ovary ; LT., uterus ; G., gut. numerous laterally. The epidermis makes and remakes the cuticle, which is periodically moulted, (c) Beneath the epidermis is a layer of remarkable muscle cells, lying in groups defined by the lines mentioned above. Many of the Nematodes are very agile. Around the pharynx there is a nerve-ring from which six nerves run forwards and six backwards. One runs along the median dorsal line — a unique position in an 238 UNSEGMENTED WORMS Invertebrate. Here and there on the ring and on the nerves there are ganghonic cells, but there is but little aggregation of these into ganglia. Sense organs are represented by the papillae already mentioned. As the food consists of juices from a living host, it is not surprising to find that the alimentary canal has but a narrow cavity. It consists of three parts — a fore-gut or oesophagus, lined by the inturned cuticle, a mid-gut or mesenteron of endodermic origin, and a usually short hind-gut or rectum lined by the cuticle. When the external cuticle is shed, so is that of the fore- gut and hind-gut (cf. Crayfish). There is a distinct space between gut and body wall, but it is lined externally by the muscle cells, internally by the endoderm of the gut, which has no mesoblastic coat ; the space is therefore not strictly coelomic. It contains a clear fluid, which probably discharges some of the functions of blood. There are no free amoeboid phagocytes. Embedded in each lateral line there is a longitudinal canal. These unite anteriorly, and open in a ventral excretory pore near the head. They seem to be associated internally with fixed phagocytic cells. In the species dis- cussed there are four giant branched cells situated anteriorly, which are especially con- nected with taking up waste particles. The relation of this excretory system to that of other Invertebrates is unknown. The sexes are separate. In the male the testis is unpaired — a coiled tube Fig. 130. — Diagram of the structure of a male Nematode. M., Mouth; CE., oesopha- gus; GA., nerve ring; B., bulb at lower end of fore-gut ; G., mesen- . _p - . . teron ; SP., spme with gradually difterentiatmg mto vas de- ejSuiatory 'duett Vs. "i fcrcus, scmiual vcsiclc, and ejaculatory seminal vesicle;' T.', duct. The genital apcrturc is close to testis ' £, i , ion*^itucli- O 1 nai excretory tube, cut the auus. The spcrmatozoa have not short; EP., excretory ^^^ typical form, and are sluggish. Their movement within the female ducts appears to be due to flagella-like villous processes from the walls. In the female the ovary is a paired tube, pore. NEMATODES 239 which passes gradually into an oviduct and a uterus at each side, and a short unpaired vagina. The genital aperture is ventral and anterior. The ova meet the spermatozoa at the junction of uterus and oviduct. Segmentation is total, and resuhs in the formation first of a blastula and then of a gastrula. The germ-cells are distinguishable very early from the body- cells. Blastopore and archenteron are obliterated, the mid-gut arising as a secondary splitting between two rows of endoderm cells. The eggs pass out of the gut of the host and probably hatch in water, and are thus re-intro- duced. No intermediate host is known. There is evidence that the larvae of Ascaris in some hosts exhibit an extensive migration within their host before settling down to mature in the intestine. The same may be true in man. Though parasitism is exceedingly common among Nematodes many are free-Hving for at least a part of the life-cycle, and feed on putre- fying organic matter. Although the number of individuals which may infest one host shows how successful the parasitism is, yet Nematodes exhibit few of the ordinary adaptations to a parasitic Ufe, and there is no sharp structural line of demarcation between free and parasitic forms. Some, like Ascaris, secrete an irritating toxin. Among histo- logical pecuharities, the practically complete absence of cilia — paralleled elsewhere only among the Arthropods — the nature of Hhe muscle-cells, the condition of the subcuticular layer, are to be noticed. Among the grosser structural pecuharities, the nature of the excretory system, of the cavity of the body, and of the nervous system, are worthy of special note. Sense organs are never well developed, but in the free-living forms simple eyes may occur. The alimentary canal is usually completely developed, but may, as in Sphcsrularia, be degenerate. Of the relationships nothing is known. Life-Histories 1. The embryo grows directly into the adult, and both live in fresh or salt water, damp earth, and rotting plants— Enoplida, e.g. Enoplus. 2. The larvce are free in the earth, the sexual adults are parasitic in plants, or in Vertebrate animals, e.g. Tylenchus scandens, a common parasite on cereals ; Strongyhis and Dochmius in man. 3. The sexual adults are free, the larvae are parasitic in insects, e.g. Merniis. The fertilised females of Sphcerulana bombi pass from the earth into the body cavity of humble-bee and 240 UNSEGMENTED WORMS wasp, whence their larvae bore into the intestine and eventually emerge. 4. The larvae are parasitic in one animal, the sexual adults in another which feeds on the first. Thus Ollulanus passes from mouse to cat, Cucullanus from Cyclops to perch. There are other life-histories, and many degrees of parasitism. The most remarkable form is Angiostomum (or Ascaris or Leptodera) nigrovcnosum. In damp earth males and females occur, the progeny of which pass into the lungs of frogs and toads. There they mature into hermaphrodite animals (the only example among Nematodes), which produce first spermatozoa and then ova. They are self-impregnating, and the young pass out into the earth as males or females. Here there is alternation of generations : and a somewhat similar story might be told of Rhahdonema strongyloides from the intestine of man, and Leptodera appendiculata from the snail. There are several quaint reproductive abnormalities, thus — the female SphcBvularia bonibi, which gets into the body cavity of the humble-bee, has a prolapsed uterus, larger than itself : the male of Trichodes crassicauda passes into the uterus of the female. Parasitic Nematodes Trichinella (Trichina) spiralis is a formidable parasite in man, pig, and rat, but it has been introduced experimentally into hedgehog, fox, dog, cat, rabbit, ox, and horse. The sexual forms live in the intestine, the female about 3 mm. in length, the male less than half as long. After impregnation the female brings forth numerous embryos viviparously, sixty to eighty at a time, and altogether about 1500. These are produced in the wall of the intestine, or in the adjacent lymphatic spaces. Most of them find their way into lymph and blood vessels, and are swept by the blood stream to the muscles ; occasion- ally some seem to migrate actively, boring their way especially through connective tissue. The migration causes inflammation and fever. In the muscle fibres they grow, coil themselves spirally, and become encysted within a sheath, at first membranous and afterwards cal- careous (Figs. 131 and 132). The cyst is partly due to the muscle, and partly to the parasite. The infected muscle fibre degenerates. In these cysts, which may be sometimes counted in millions, the young Trichinae remain passive, unless the flesh of their host be eaten by another — pig eating rat, man eating pig. In the ahmentary canal of the new host the capsule is dissolved, the embryos are set free, and become in two or three days reproductive. The male seems to die after copulation. Among the numerous other parasitic Nematodes the following may be noted : — The giant pahsade worm {Eustrongylus gigas) occurs in the renal region of domestic animals, etc. ; the female may be 3 ft. long. The armed palisade worm {Strongylus armatus) occurs in the intestine and intestinal arteries of horse, causing aneurysms, colic, etc. The THREAD- WORMS 241 young forms are swallowed from stagnant water, bore from gut into arteries, become adult, return to gut, copulate and multiply. Various other species of Strongylus occur in sheep, cattle, etc. Of the genus Ascaris alone, over 200 species have been found in all types of Verte- brates : — A. megalocephala in horses, A. lumbricoides in man, A. mystax in cats and dogs. Syngamus trachealis occurs in the trachea of birds, gapes," e.g. in poultry and pheasants. It pierces the wall causmg Fig. 131 . — Trichinae in miiscle, about to be encapsuled. — After Leuckart. Fig. 132. — Trichinae in muscle, encapsuled. There may be 12,000 in a gramme of pig's muscle. — After Teuckart. of the trachea, and " actually clenches the teeth with which its mouth is provided in the tracheal rings." A remarkable large form, Ichthyo- nema grayi, is found inside sea-urchins. Various species of Tylenchus, especially T. devastatrix and T. scandens (or T. tritici), destroy cereal and other crops. Various species of Heierodera (especially H. schachtii and H. radicicola) infest the roots of many cultivated plants, e.g. turnip, radish, cabbage. There is evidently a great variety of habit and habitat among Nematodes, and yet the general structure is very uniform. They seem to represent a homogeneous class, very much by themselves, and not nearly related to other types, even to the other Nematohelminthes. 16 242 UNSEGMENTED WORMS Some of the most Important Forms Parasitic in Man Name. Position. i History. Kesvlt on Host. A scaris lumbri- Usually in small Repeated experi- Conuiionest in coides, maw-worm intestine. ment has shown that children ; rarely (common). infection results if dangerous, unless [A . mystax, com- the eggs (with their very numerous, or mon in dogs and outer envelope en- maturing in other cats, has also been tire) are swallowed parts of the body, found in man. J along with vegetable e.g. respiratory food or otherwise. tract, bile duct, .\fter hatching, the vermiform appen- larva? may be dis- dix. Like others, tributed in the blood it may puncture stream. the wall of the gut and liberate patho- From food or genic bacteria. Oxyuris vermi- Frr.ni stomach Rarely more than cular is (common). to rectum, mostly water. discomfort. in colon. Trichocephalus Colon ; more dispar or trichi- rarely appendix iirus, the whip- and small intestine. worm (common). A nchylostomum Small intestine. The larva^ live Ulceration, hae- diiodenaie (widely freely in the earth. morrhage,and dan- distributed). Infection usually gerous anaemia. Necator, another through the skin. Serious sapping of closely related vitality in warm " Hookworm." Filaria bancrofti Mature female countries. Larvae in a mos- Elephantiasis and (Australia, China, (80-100 mm.) in quito, haematuria. | India, Egypt, and lymphatic glands, Brazil). embryos in blood. .Males rare (30-45 mm.). Dracuncuhis (Fil- The female is 1-6 Larvae in a Cy- Subcutaneous aria) medinensis ft. long, encysts clops, abscesses. | (Guinea-worm), in beneath skin, es- .Arabia, Egypt, pecially of back or Abyssinia, etc. legs. Male rarely seen. Trichinella spir- Becomes sexually From " trichi- Inflanunatory pro- lii':, widely dis- mature in the in- nosed " pig's mus- cesses, often fatal, tributed. testine ; embryos. cle to man. are brought about produced rapidly by the migration and viviparously. of the young worms find their way to [ from intestine to muscles, and be- muscles. 1 come encysted. 1 1 CLASSES OF NEMATOHELMINTHES 243 Class Nematomorpha The Gordiidae (e.g. Gordius aquaiicus — the horse-hair worm) are so different from true Nematodes that they must be ranked in a separate class. There are no lateral lines. Three nerve-strands lie close together in the mid-ventral line. In the adult Gordius the mouth is shut and the food canal is partly degenerate. The adult Gordiidae usually live freely in fresh water ; larval forms occur in aquatic molluscs, young insects, etc. ; later stages usually occur in carnivorous insects, whence they emerge to become adult in the water. One form, Nectonema agile, is marine. Class Acanthocephala For a few genera, of which the best known is' Echinorhynchus, whose larvae live in Arthropods, and the adults in Vertebrates, a special class, Acanthocephala, has been established. They may be placed beside Nematodes, but the relationship does not seem to be very close. Mouth and gut are absent. The anterior end bears a protrusible hooked proboscis used in boring in the intestinal wall of the host. In the minute swellings at the ends of the two much-branched excretory organs of E. gigas, there are ciliated cells — the only case known among Nematohelminthes. Echinorhynchus proteiis of pike, minnow, trout, etc., larva in the Amphipod Gammarus pulex. „ angustafus of perch, larva in the Isopod Asellus aquaticus. „ moniliformis of rat, etc., larva in larval beetles {Blaps). „ gigas of pig, larva in grubs of cockchafer, etc. Desiccation Many of the smaller Nematohelminthes are able to survive prolonged drying up or desiccation. The body may become quite brittle, and yet replacement in water brings about revivification — even after years. This state of latent life is of great theoretical interest, for the living matter loses most of its water-content and passes out of the colloid state. CHAPTER XI PHYLUM ANNELIDA Chief Classes — Ch^topoda, Hirudinea or Discophora The Annelids or Annulata are segmented worms, in most of which the segmentation of the body is visible externally. The head usually consists of a pre-oral " prostomium " and a post-oral peristomium. The body wall has several layers of muscles , and many, e.g. Chcetopods, have setce embedded in the skin. In most, there is a well-developed caelom, communicating with the exterior by paired nephridia. The nervous system consists typically of two dorsal cerebral ganglia, a commissural ring round the gullet, and a ventral ganglionated chain. The gonads arise on the coelomic epi- thelium. Not infrequently the nephridia function also as genital ducts. The development may be direct or indirect, and if indirect it usually includes a larval Trochosphere stage. In habit, form, and structure the AnneUds exhibit much diversity. The Chaetopods, represented by the famihar earthworms and by the marine worms, are most typical. With these may be included the aberrant Echiuridae, e.g. Echiurus and Bonellia. A few primitive forms (Archi- Annelida), and the Myzostomata (parasitic on Crinoids), may also be appended. The leeches (Discophora) are divergent. Further, some zoologists include Chaetognatha in this series as Annelids with three segments, and also the Rotifers (Rotatoria), whose adult form somewhat resembles the Trochosphere larvae of many Annelids. Finally there are associated in an uncertain way Sipunculids, Gephyreans, Phoronids, Polyzoa, and Brachiopods. 244 BRISTLE-FOOTED WORMS Class Ch^topoda. Annelids with Bristles 245 Segmented animals with setce developed in little skin-sacs, either on a uniform body wall or on special locomotor pro' trusions known as parapodia. The segments, indicated externally by rings, are often marked internally by parti- tions running across the body cavity, which is usually well developed. The nervous system generally consists of a double ventral chain of ganglia, connected with a pair of dorsal cerebral ganglia by a ring round the beginning of the gut. Two excretory tubes or nephridia are typically present in each segment, and they or their modifications may also function as reproductive ducts. The reproductive elements are formed on the lining membrane of the body cavity. The development is either direct or with a metamorphosis. Type of Oligoch^ta. The Earthworm (Lumbricus) Habits. — Earthworms eat their way through the ground, and form definite burrows, which they often make more comfortable by a Hning of leaves. The earth swallowed by the burrowers is reduced to powder in the gut, and, robbed of some of its decaying vegetable matter, is discharged on the surface as the familiar " worm-castings." By the burrowing the earth is loosened, and ways are opened for plant-roots and rain-drops ; the internal bruising reduces mineral matter to more useful form ; while, in covering the surface with earth brought up from beneath, the earth- worms have been ploughers before the plough. Darwin calculated that there were on an average over 53,000 earthworms in an acre of garden ground, that 10 tons of soil per acre pass annually through their bodies, and that they cover the surface with earth at the rate of 3 in. in fifteen years. He was therefore led to the conclusion that earthworms have been the great soil-makers, or, more precisely, that the formation of vegetable mould was mainly to be placed to their credit. Though without eyes, earthworms are sensitive to light and persistently avoid it, remaining underground during the day, unless rain floods their burrows, and reserving 246 PHYLUM ANNELIDA their active life for the night. Then, prompted by " love " and hunger, they roam about on the surface, leaving on the moist roadway the trails which we see in the morning. smmm^;v \f 'i o -t-> l-l CO O More cautiously, however, they often remain with their tails fixed in their holes, while with the rest of their body they move slowly round and round. The nocturnal peregrina- tions, the labour of eating and burrowing, the transport of STRUCTURE OF EARTHWORM 247 leaves to their holes, the collection of little stones to pro- tect the entrance to the burrows, include most of the activities of earthworms, except as regards pairing and egg- laying, of w^hich something will afterwards be said. When an earthworm is halved with the spade, it does not neces- sarily die, for the head portion may grow a new tail, while a decapitated worm may even grow a new head and brain. Phagocytes help as usual in the regeneration. The earth- worm is much persecuted by numerous enemies, e.g. centi- pedes, moles, and birds. The male reproductive organs are always infested by uni- cellular parasites — Gregar- ines of the genus Monocystis ; and minute thread - worms (Pelodera pellio) usually occur in the nephridia and body cavity, and often in the ventral blood vessels. Form and external characters. — The earthworm is often about 6 in. long, with a pointed head end, and a cylindrical body rather flat- tened posteriorly. The successive rings seen on the surface mark true segments. The mouth is overarched by a small lobe called the pro- stomium, and the food canal ter- minates at the blunt posterior end. The skin is covered by a thin trans- parent cuticle, traversed by two sets of fine lines, which break up the light and produce a slight iridescence. On a region extend- ing from the 31st to the 38th ring, the skin of mature worms is swollen and glandular, forming the clitellum or saddle, which helps the worms, as they unite in pairs, and also forms the slimy stuff which hardens into cocoons. The middle line of the back is marked by a special redness of the skin. On the sides and ventral surface we feel and see four rows of tiny bristles or setaj, which project from little sacs, are worked by muscles and assist in locomotion. These bristles are fixed like pins into the ground, at times so firmlv that even a bird finds it difficult to pull the worm from its hole. " As each of the four longitudinal rows is double, there are obviously eight bristles to each ring. On the skin of the ventral surface there are not a few special apertures, which should be looked for on a full-grown worm ; but careful examination of several specimens Fig. 134. — Anterior region of earthworm. — After Hering. Note the eight setaj (s.) on each segment. R.S., Spots between 9-10, lo-ii. indicate openings of receptacula seminis ; Ovd., openings of oviducts on segment 14 ; y.^., openings of vasa deferentia on segment 15. 248 PHYLUM ANNELIDA is usually necessary. Almost always plain on the 15th ring are the two swollen lips of the male ducts, less distinct on the 14th are the apertures of the oviducts through which the eggs pass, while on each side, between segments 9 and 10, 10 and 11, are the openings of two receptacula seminis or spermotheca; into which male elements from another earthworm pass, and from which they again pass out to fertilise the eggs of the earthworm when these are laid. Each segment con- tains a pair of excretory tubes, which have minute ventral-lateral apertures, while on the middle line of the back, between the rings, there are minute pores, through which fluid from the body cavity may exude on to the skin. Skin and bristles. — The thin cuticle is produced by the cells which lie beneath, and is perforated by the apertures previously mentioned. The epidermis clothing the worm is a single layer of cells, of which most are simply supporting or covering elements, while many are slightly modified, as glandular or mucous cells, and as nervous cells. As the latter are connected with afferent fibres which enter the nerve-cord, the skin is diffusely sensitive. In a few species the skin is slightly phosphor- escent. The chitinous bristles, which are longest on the genital segments, are much curved, and lie in small sacs of the skin, in which they can be replaced after breakage. Muscular system and body cavity. — The earthworm moves by the contraction of muscle cells, which are arranged in circular hoops and longitudinal bands under- neath the skin. The special muscles about the mouth and pharynx have considerable powers of grasping, while less obvious muscular elements occur in the wall of the gut, in the partitions which run internally between the segments, and on the outermost portions of the excretory tubes. Unlike the leech, the earthworm has a very distinct body cavity, through the middle of which the gut extends, and across which run the partitions or septa incompletely separating successive segments. In this cavity there is some fluid with cellular elements, of which the most numerous are yellow cells detached from the walls of the gut. Possible communications with the exterior are by the dorsal pores, and also by the excretory tubes, which open internally into the cavities of the segments. Nervous system. — ^Along the middle ventral line lies a chain of nerve-centres or ganglia, really double from first NERVOUS SYSTEM OF EARTHWORM 249 to last, but compactly united into what to unaided eyes seems a single cord. As the segments are very short, the limits of the successive pairs of ganglia are not very evident, especially in the anterior region, but they are plain enough on a small portion of the cord examined with the micro- scope, when it may also be seen that each of the pairs of ganglia gives off nerves to the walls of the body. Anteriorly, just behind the mouth, the halves of the cord diverge and ascend, forming a ring round the pharynx. They unite above in two dorsal or cerebral ganglia, which are situated in the peristomium or first ring, and not, as in Polychaetes, in the prostomium. These form the earthworm's " brain," and give off nerves to the adjacent pre-oral lobe or pro- stomium, on which are numerous sensitive cells. These, coming in contact with many things, doubtless receive impressions, which are transmitted by the associated nerves to the " brain." As Mr. Darwin observed that earthworms seized hold of leaves in the most expeditious fashion, taking the sharp twin leaves of the Scotch fir by their united base, we may credit the earthworms with some power of profiting by experience ; moreover, as they deal deftly with leaves of which they have no previous experience, we may even grant them a modicum of intelligence. From the nerve- collar uniting a dorsal ganglia with the first pair on the ventral cord, nerves are given off to the pharynx and gut, forming what is called a " visceral system." The earth- worm has no special sense organs, but there are abundant sensitive cells, especially on the head end. By them the animal is made aware of the differences between light and darkness, and of the approaching tread of human feet, not to speak of the hostile advances of a hungry blackbird. The sense of smell is also developed. The afferent or sensory nerve fibres from the nervous cells of the skin enter the nerve-cord and bifurcate into longitudinal branches, which end freely in the nearest ganglia. In this the earth- worm's nervous system suggests that of Vertebrates. The nerve cells, instead of being confined to special centres or ganglia, as they are in Arthropods, also occur diffusely along with the nerve fibres throughout the course of the cord. Along the dorsal surface of the nerve-cord there run three peculiar tubular " giant fibres,"' with firm walls and clear contents. They are probably comparable to the medullated nerve fibres of Vertebrates. 250 PHYLUM ANNELIDA Alimentary system. — Earthworms eat the soil for the sake of the plant debris which it may contain, and also because one of the modes of burrowing involves swallowing the earth. In eating they are greatly helped by the muscular nature of the pharynx ; from it the soil passes down the gullet or oesophagus, first into a swollen crop, then into a strong-walled grinding gizzard, and finally through a long digestive and absorptive stomach-intestine. There are three pairs of oesophageal glands. Canals from the posterior two pairs open into the anterior pair, and thus into the gullet. Their contents are Hmy, and perhaps counteract the acidity of the decaying vegetable matter. It may be that they are in part excretory ; or it may be that they serve to fix some of the carbon dioxide formed by the animal. The long intestine has its internal surface increased by a dorsal fold, which projects inwards along the whole length. In this " typhlosole," and over the outer surface of the gut, there are crowded yellow cells. There is no warrant for calling the yellow cells hepatic or digestive. Structurally they are pigmented cells of the peritoneal epithelium, which here, as in most other animals, lines the body cavity and covers the gut. As to their function, they absorb particles from the intestine, and go free into the body cavity, whence, as they break up, their debris may pass out by the excretory tubes. When a worm has been made to eat powdered carmine, the passage of these useless particles from gut to yellow cells, from yellow cells to body cavity, and thence out by the excretory tubes, can be traced. The amoeboid cells of the body cavity fluid act as phagocytes. Various ferments have been detected in the gut, a diastatic ferment turning the starchy food into sugars, and others — peptic and tryptic — not less important. The wall of the stomach-intestine from without inwards, as may be traced in sections, is made up of pigmented peritoneum, muscles, capillaries, and an internal ciliated epithehum. In the other parts of the gut the innermost lining is not ciliated, but covered with a cuticle. Vascular system. — The fluid of the blood is coloured red with haemoglobin, and contains small corpuscles. Along the median dorsal line of the gut a prominent blood vessel extends, another (supra-neural) runs along the upper surface of the nerve-cord, another (infra-neural) along the under surface, while two small latero-neurals pass along each side of this same cord. All these longitudinal vessels, of which the first three are most important, are parallel with one another ; the first three meet in an anterior net- EXCRETORY SYSTEM OF EARTHWORM 251 work on the pharynx ; the dorsal and the supra-neural are linked together in the region of the gullet by five or six pairs of contractile vessels or " hearts." Respiration is efl[^ected by the distribution of blood on the general surface of the skin. Excretory system. — There is a pair of nephridia in each segment except the first four. Each opens internally into Fig. 135. — Transverse section of earthworm. .4., Cuticle; B., epidermis; CM., circular muscles; L.M., longitudinal muscles ; D., a seta ; C, coelom ; Y.C., yellow cells ; F., typhlosole ; V.V., supra-neural blood vessel ; S.V., sub-neural vessel ; DA' ., dorsal vessel; P., peritoneum ; £., cavity of gut ; G., endodermiclining of gut ; N., part of a nephridium ; R., opening of a nephridium ; /., the nerve- cord ; /., a nerve given off ; K., giant fibres in the nerve-cord. the segment in front of that on which its other end opens to the exterior. They remove little particles from the body cavity, and get fluid waste products from the associated blood vessels. Nephridia occur in many animals, in most young Vertebrates as well as among Invertebrates, but thev are never seen more clearly than in the earthworm. When a nephridium is carefully removed, along with a part of the septum through which it passes, and examined under 252 PHYLUM ANNELIDA the microscope, the following three parts are seen : — (a) An internal ciliated funnel ; (b) a trebly coiled ciliated tube, at first transparent, then glandular and granular ; and (c) a muscular duct opening to the exterior. Minute par- ticles swept into the ciliated funnel pass down the ciliated coils of the tube, and out by the muscular part which opens just outside of the ventral bristles. The coiled tube con- sists in part at least of a series of intracellular cavities, that is to say, it runs through the middle of the cells which compose it ; the external muscular portion arises from an invagination of skin. Reproductive system. ^ — Like all Oligochaetes,the earth- worm is hermaphrodite and the organs complex. The complexity is produced by the specialisation of certain of the nephridia to form genital ducts and accessory organs, and by the presence of chambers (seminal vesicles) con- nected with the testes, formed by the shutting off of portions of the body cavity. The organs in the earthworm are difficult to dissect, and differ considerably in old and young specimens. (a) The Male Organs consist of two pairs of testes, three pairs of seminal vesicles, and paired vasa deferentia. (i) The testes, flattened lobed bodies, about ^\, in. in size, arise from proliferations of the peritoneal lining of the body cavity, and are invested by a delicate membrane derived therefrom ; they lie near the nerve-cord, attached to the posterior surfaces of the septa between segments 9-10 and lo-ii. They are minute, translucent, and difficult to see. In immature worms they lie exposed in the body cavity ; in mature worms they are concealed by the great development of the seminal vesicles. (2) The seminal vesicles are much-lobed structures, exceedingly prominent in dissection. Small and laterally placed in young worms, in the adult the anterior two pairs fuse in the middle line and cover the anterior pair of testes and their ducts, while the posterior pair similarly conceal the second pair of testes with their ducts. Into the seminal vesicles mother sperm cells from the testes pass, and divide up to form spermatozoa. Development shows that the seminal vesicles arise as outgrowths of the septa of segments 9-12, and that their REPRODUCTIVE SYSTEM OF EARTHWORM 253 lumen is a portion of the body cavity. This is of import- ance, for in Polychaetes the genital products mature in the general body cavity, just as the spermatozoa in the earth- worm mature in the seminal vesicles. (3) From the seminal vesicles the spermatozoa are carried to the exterior by means of the vasa deferentia. The in- ternal openings of these are large and funnel-shaped, and are concealed by the seminal vesicles. Each of the four 0Vd4 Fig. 136. — Reproductive organs of earthworm. - After Hering. N., Nerve-cord ; T., anterior testes ; S., sacs of setse ; R.S., receptacula seminis ; s.f., seminal funnels ; v.d., vas deferens ; ovd., oviduct ; ov., ovary ; s.v., seminal vesicles cut open ; VIII.'XV., segments. funnels opens into a duct, and the two ducts unite at each side to form the two elongated vasa deferentia, which pass backwards to open externally on the 15th segment. (b) The Female Organs consist of two ovaries and two oviducts, each of which has a side receptacle for the eggs. (i) The two ovaries are small bodies situated near the nerve-cord on the septum between segments 12-13. Each is pear-shaped, the stalk of the pear being a string of ripe ova. They are more readily seen than the testes. 254 PHYLUM ANNELIDA (2) The two oviducts open internally on the anterior face of the septum between 13-14, and externally on the ventral surface of segment 14. Into the wide ciliated internal mouths, which lie opposite the ovaries, the ripe eggs pass. (3) The egg-sac or receptaculum ovorum, near the internal mouth of each oviduct, is a posterior diverticulum of the septum between segments 13-14. Within it a few mature ova are stored. (c) Two pairs of receptacula seminis or spermathecae receive spermatozoa from another earthworm, and liberate them, to fertilise the eggs of this one. They are white globular sacs, opening in the grooves between segments 9-10 and lo-ii, and probably, like the genital ducts, arise from modified nephridia. According to some, these sper- mathecae not only receive and store spermatozoa, but make them into packets or spermatophores. Others say that the glands of the clitellum make these packets. At any rate, minute thread-like packets of spermatozoa are formed, and a pair of them may often be seen adhering to the skin of the earthworm about the saddle region. When two worms unite sexually, they lie with the head of the one towards the tail of the other, with about 40 segments overlapping. The spermatozoa of the one pass along a groove into the receptacula of the other. When the eggs of an earthworm are hberated, they are surrounded by a gelatinous sheath secreted b'y the saddle. As this is peeled off towards the head, spermatophores are also enclosed. When free, the ends of the sheath close, and a lemon-shaped cocoon results. Development. — Many cocoons are made about the same time, and each contains numerous ova, and also packets of sperms, so that fertilisation takes place outside the body. These cocoons are buried in the earth a few inches below the surface. They measure about a quarter of an inch in length. The favourite time for egg-laying is during the, spring and summer, though it may be continued throughout the whole year. The earth- worm of the dungheap (L. fcetidus) makes this a habit, induced probably by the warmth of its environment. Of the many ova in the cocoon of L. terrestris, only one comes to maturity, while in L. fcetidus a few, and in L. communis two may do so. But in the last species the two embryos are often twins formed from one ovum, separation taking place at the gastrula stage. The whole process of growth, until leaving the egg, lasts from two to three weeks, the time varying, however, with the temperature. The ovum is surrounded by a vitelUne membrane, and is laden with yolk granules. Segmentation is slightly unequal (Fig. 137 (i)), and DEVELOPMENT OF EARTHWORM 255 exhibits considerable variation even within the hmits of a species. In about twenty-four hours a nearly spherical, one-layered blasto- sphere or blastula is formed. It con- sists of only about thirteen cells. During the next twenty-four hours the cells increase in number rapidly, but the blastula remains one-layered. Two cells lying together do not take part in this division ; they are rather larger than the rest, and their inner ends project into the cavity, and are soon cut off as daughter-cells. Gradu- ally the large cells still undergoing division begin to sink in, and at last are quite included in the cavity (Fig. 137 (2)). Thus there arise two parallel rows of cells within the blastula, and these define the longitudinal axis of the embryo. This is the beginning of the mesoblast which forms all the muscles of the trunk, and which thus takes origin from two primary meso- blasts. After five to six pairs of secondary mesoblasts have been formed, the blastula begins to flatten, and to elongate, becoming an oval disc. The cells of the lower surface be- come clearer, and the endoderm is thus defined. The cells of the upper surface are smaller, and become very much flattened ; they compose the ectoderm. The mesoblasts lie side by side near one end, forming two rows extending forwards and down- FiG. 137. — Stages in the develop- ment of earthworm. — After Wilson. I. 2. 3- Two-celled stage ; p.c, polar bodies. Blastula ; M., primary mesoderm cell. Gastrula stage ; Ec, ectoderm ; En., endoderm in process of being covered by the small ectoderm cells. Note the widely open blastopore ; M., mesoderm cells. Longitudinal section in late gastrula stage, showing germ-bands ; ec, ectoderm ; en., endoderm ; M., mouth ; St., stomo- daeum ; m., primary mesoderm cells; Nb., neuroblasts ; n.c, nerve-cord ; N., nephridioblasts ; ms., mesoderm bands ; npc, incipient nephridia. ms ^^m 256 PHYLUM ANNELIDA wards, but divergent, because of the flattening of the blastula. The endodenn now becomes concave, and thus the blastopore arises, occupying the whole of the lower surface (Fig. 137 {3)). The sides close in and the blastopore becomes a slit, which further closes from behind forwards, leaving only a small opening — the future mouth. During these processes the cells at the anterior tip of the blastopore, which will give rise to the pre-oral lobe, undergo no change, but the mesoderm has been active. As gastrulation proceeds, the mesoblast rows grow forwards and upwards, until they come near each other above the anterior tip of the blastopore, while their middle portions are carried downwards until the}' lie on the ventral surface. Over them the ectoderm is thickened in two bands. Two longitudinal rows of ectoderm cells near the anterior end, and ending behind in large cells, sink in just as the primary mesoblasts did. The thickening now extends ventrally until the two bands meet and, passing into the blastopore, form the stomoda^um. Even before this the embryo has begun to swallow the albumin in which it floats. There are now two lateral bands of cells called the germ-bands, composed of three layers (Fig. 137 (4)) ; outside is the thickened ecto- derm, next the rows of cells which sank in, and innermost the meso- blast rows. The mesoblast rows have met in the middle line by dividing and widening out into a pair of flattened plates, but they still end behind in the two primary mesoblasts. Coelomic cavities develop in the plates, and the anterior ends meet above the mouth. The ectodermic rows which sank in (there were eight of them, four on each side of the median line, and each ending in a large mother-cell) go on growing. The mother-cells are apparently carried backwards as the embryo lengthens, leaving a trail of daughter-cells behind them. The cells so formed also divide, the embryo rapidly lengthening and finally becoming vermiform. Of the eight rows the innermost on each side (neuroblasts) give rise to the nervous system, the next two rows on each side (nephridioblasts) form parts of the nephridia (Fig. 137 (4)), while of the fourth row nothing definite is known. Each row, ending behind in a single cell, widens out and deepens as it is traced forwards. The neural and mesoblastic rows can be traced round the mouth, and help to form the prostomium ; the others fade away at the sides of the stomodaeum. Let us sum up this complex history : — ''(a) The original outer layer becomes the epidermis. (b) The secondary inner strat- um consists of neuroblasts Ectoderm. • which form the nervous system, of nephridioblasts which form parts of the nephridia, and of lateral cells of unknown function. Fertilised oviun. Blastula. Two-layered gastrula with primitive mesoderm cells. Mesoderm fonned from the division of the primitive " mesoblasts." /"Muscle. I Blood vessels. "j Inner parts of nephridia. ' Reproductive organs. Endoderm. / Lining of \ mid-gut. POLYCH^TES 257 Type of PoLYCH^zETA. The Lob-worm (Arenicola marina) Habits. — On the flat sandy beach uncovered at low tide, the " castings " of the lob-worm or lug-worm are very numerous. There the fishermen seek the worms for bait, and have to dig quickly, for the burrowers retreat one to two feet into the sand. The burrows are curved tubes, lined by a yellowish green secretion from the animal's epidermis, and the surrounding sand is often discoloured by some change which the secretion eff'ects on the iron oxides and other constituents. The tubes are at first vertical. Fig. 138. — Arenicola marina. Entire animal viewed slightly from left side. Note anterior jnouth ; setae on anterior region ; setae and gills on median region ; thinner tail region often longer than shown. afterwards oblique or horizontal, and then turn vertically upwards again. The lob-worm burrows like the earthworm, not only forcing the anterior part of its body onwards, but eating the sand for the sake of the organic particles and small organisms which it contains. The sandy castings, which pass from the end of the food canal, and are got rid of at the mouth of the tube, fall into spiral coils. It has been calculated that in a year the average volume of sand per acre thus brought up in castings is about 1900 tons, representing a layer of 13 in. spread out over the surface. This work, comparable to that of earthworms, tends to cleanse the sand and to reduce it to a finer powder. When getting rid of the casting, the worm lies with its tail upwards and its head downwards, or with its body bent like a bow ; when the tide comes in, the mouth may protrude at the other end of 17 258 PHYLUM ANNELIDA the U-shaped tube. The worms that live between tide- marks seem to differ in many respects (as to colour, gills, habits, and sexual maturity) from those which occur in the Laminarian zone, which is only uncovered at low spring- tides. External appearance. — The lob-worm varies in length from 8 to 16 inches, and at its thickest part is about half an inch in diameter. There are three regions in the body : (a) The anterior seven segments, of which all but the first have bristles ; (b) the middle region of thirteen segments, with both gills and bristles ; (c) the thinner posterior part of variable length, without either gills or bristles, and with an inconstant number of segments (up to about thirty). In the very front there is a head-lobe or prostomium, but there are no tentacles or eyes. Anteriorly a soft proboscis is often protruded from the gut. The anus is terminal. Skin, muscles, and appendages. — Each segment is marked by about four superficial rings. The epidermis is pigmented and secretes mucus, and is divided into numerous polygonal areas, separated by shallow grooves. Beneath the epidermis is a sheath of circular muscles, and then a layer of longitudinal muscles. Besides these there are (from the middle of the gullet to the beginning of the tail) thin oblique muscles arising from the sides of the nerve-cord, and dividing the body cavity longitudinally into a central and two lateral compartments. Other muscles control the prostomium, the proboscis, and the bristles. Unlike many of the marine Annelids, Arenicola has very rudimentary appendages. This reduction of appendages must be associated with the animal's mode of life ; it occurs also in many tube-inhabiting worms. Neither the prostomium nor the first segment shows any trace of appendages, but the next nineteen have rudiments. The dorsal part (notopodial) consists of a tuft of bristles, whose bases are enclosed in a sac ; — the ventral part (neuropodial), separated by a short interval, bears several hooks. Nervous system. — This is in its general features like that of the earthworm, but ganglia are not developed. In the ventral nerve-cord, the ring round the gullet, and the slight cerebral enlargement which represents a brain, STRUCTURE OF LOB-WORM 259 nerve cells occur diffusely scattered among the nerve fibres. Along the dorsal surface of the nerve-cord, in the branchial region, there are two " giant fibres " like those in the earthworm ; anteriorly and posteriorly there is only one. The prostomial lobes are diffusely sensory, and bear also two ciliated, probably olfactory, pits — the "nuchal organs." Otherwise sense organs are represented only by a pair of otocyst sacs (Fig. 139), one on each side of the oesophageal nerve-ring. These sacs, like those which occur in many other Invertebrates, seem to have to do rather wdth the direction of the animal's movements than with hearing. Professor Ehlers notes an interesting series : In .-1 . claparedii there are simply two open grooves ; in ^ . marina the sacs have open necks, and contain foreign par- ticles ; in ^. grubii and A. antillensis the sacs are closed, and contain intrinsic oto- liths of lime. ^Ot. The as a which buccal '' pro- grips Food canal. — (i) cavity is protrusible boscis " or introvert the sand, and bears internal papillae with chitinous tips. The protrusion is due to the pressure of the ccelomic fluid, while special muscles bring about retraction. (2) The gullet has smooth walls, and bears a posterior pair of glands, w'hich secrete a yellowish fluid, probably digestive. (3) The gastric region, from the heart to the twelfth or thirteenth notopodium, is covered many blood vessels, and has a groove -ce.r. Fig. 139. — Anterior part of nervous system in Arenicola. — After Vogt and Yung. c, Cerebral part on dorsal sur- face ; ce.r., cesophageal ring ; g., gullet ; v.n.c, ventral nerve- cord ; /.«., lateral nerves; ot., otocyst. with yellow cells and median-ventral ciliated (4) The intestinal region is much folded, " in a concertina-hke manner," by the caudal septa, and is full of sand, from which the nutritive matter has been absorbed. The anus is at the very end. Body cavity. — This is spacious, except in the tail region, and contains a viscous ccelomic fluid. Anteriorly there are three transverse, partly muscular, septa or diaphragms which moor the gullet. The first of these diaphragms bears a pair of small pouches. Behind the third diaphragm the gut swings freely until the beginning of the tail region, in which there are many septa. 26o PHYLUM ANNELIDA Vascular system.-The blood has a bright red colour and is rich in ha3inoglobin. It flows in a very elaborate system of blood vessels, m regard to the details of which there is still some uncertainty. There is -n Fig. 140.— Dissection of lob-worm from dorsal surface. m. Opening of retracted buccal cavity ; i., gullet ; gl ., diverticula 'on^first diaphragm; gl"., oesophageal glands; rf,, ^^lo^^f ^1 ^1°^^ vessels- efh, first efferent branchial; g., stomach intest ne , n^., sixth nephriduun; t>/i3., thirteenth efferent branchial; fl/i|, thirteenth afferent branchial ; a., anus ; «/i ., first afferent branchial , h., heart of left side. STRUCTURE OF LOB-WORM 261 along the whole mid-dorsal line of the gut a contractile dorsal vessel, which carries blood fonvards from the seven posterior gills, etc. Connected with this by capillaries, there is below the gut an equally long, feebly contractile ventral vessel, which carries blood backwards to gills, nephridia, etc. Around the gastric region of the gut there is an elaborate plexus of blood vessels, which communicate by two lateral vessels with the paired heart. There are also two sub -intestinal vessels between the ventral vessel and the gut ; these lead through the plexus into the lateral gastric vessels, and thus into the hearts. These organs lie just behind the oesophageal glands, and consist on each side — [a) of a thin-walled auricle, an expansion of the lateral gastric vessel ; and (6) of a muscular ventricle, which drives the blood into the ventral vessel. Fig. 141. — Cross-section of Arenicola. — After Cosmovici. E., Epidermis ; cm., circular muscles ; l.m., longitudinal muscles ; b.c, body cavity ; gl., gill ; s., setae ; n.p., nephridial pore ; a.br., a:^erent branchial ; e.br., efferent branchial ; n., ventral nerve-cord, with blood vessels above ; d.v., dorsal vessel ; l.v., lateral vessel ; s.i.v., sub-intestinal vessels ; v.v., ventral vessel ; g-, gut. Like the sub-intestinals, the dorsal vessel communicates with the heart only indirectly through the gastric plexus. The ventricle contains a spongy " cardiac body," which probably prevents regurgitation from the ventral vessel. From the ventral vessel arise afferent branchial vessels to gills, nephridia, etc. From the seven posterior gills efferent branches enter the dorsal vessel ; while those from the six anterior gills join the sub- intestinals. Each efferent vessel gives off a branch to the skin, while the dorsal and sub-intestinal vessels give off numerous branches to the gastric plexus on the gut. Respiratory system. — There are thirteen pairs of gills, on the seventh to the nineteenth bristle-bearing segments. 262 PHYLUM ANNELIDA Each is a tuft of hollow thread-like branches, through the thin walls of which the red blood shines. The afferent branches to the gills all come from the ventral vessel ; the first six efferent vessels from the gills open into the sub- intestinals ; the posterior seven open into the dorsal vessel. As the papillae on the proboscis are hollow and contain vessels, they are doubtless of respiratory significance. Indeed, the gills may be regarded as exaggerated papillae. Excretory and reproductive systems. — In the anterior region, in segments 4-9, there are six pairs of nephridia, of which the foremost seems in process of degeneration. Each consists of three parts — a funnel opening into the body cavity, a glandular portion, and a bladder com- municating with the exterior. The sexes are separate and similar. The reproductive organs are very simple, and arise by proliferation of the peritoneal membrane beside the blood vessels supplying the funnels of the nephridia. The reproductive cells are liberated into the body cavity, and there matured. They pass out by the nephridia, and may be temporarily stored in the bladder portions of all but the first. Little is known in regard to the development, beyond the fact that the young are for a time free-swimming pelagic forms. Development of Polychaeta. — As an example of the development of the marine Cha^topods, we may take Eupomatus. Here segmenta- tion is complete, but somewhat unequal, and results in the formation of a blastula, with its upper hemisphere composed of small (ectodermic) cells, and the lower of large (endodermic) cells. Among these latter are two spherical cells — the primitive mesoderm cells, which at a much later (free-swimming) stage give rise to mesoderm bands. A gastrula is formed by invagination. Partial closure of the blastopore forms a primitive mouth and anus to the archenteron. The anus becomes closed. At a later stage the aboral region of the gastrula tilts forward, an insinking of the ectoderm forms the stomodasum, and a posterior ectodermic invagination opens to form the hind-gut and anus. The larval gut so formed has a distinct v'entral curve. Cilia appear on the surface at an early stage, and now form a distinct pre-oral ring or prototroch, and also a less constant post-oral ring or metatroch. A tuft near the anus forms a telotroch. Another tuft is formed at the apex of the pre-oral region, where an ectodermic thickening takes place ; this gives rise to an apical ganglion, with which sensory structures are often associated. Very prominent are a pair of larval excretory tubes — protonephridia — which open near the anus. The larva so formed is a typical Trochosphere, such as occurs in the great majority of Polychajta, in a more or less modified guise in many other TROCHOSPHERE LARVA 263 worm-types, and also in Mollnscs. Its chief characters are the following : — (i) There is a prominent pre-oral region, with an apical ganglion and a girdle of cilia. (2) The gut has a distinct ventral curve, and a threefold origin. Fig. 142. — Diagram showing structure of a Trochosphere of Etipomatus. — After Shearer. M., Mouth; A., anus; S., stomach; A^., larval kidney; S.O., apical sense organ ; E., eye-spot ; ME., beginnmg of mesoderm; A.V., anal vesicle. (3) The larval cavity is the persistent segmentation cavity, and in it posteriorly lie the primitive mesoderm cells. The Trochosphere is a free-swimming pelagic larva, which, among worms, corresponds largely to the future head region of the adult. The change to adult form probably takes place in the most primitive 264 PHYLUM ANNELIDA fashion in the httle worm Polygordius. We shall therefore follow it there (Fig. 143). In the larva, which is a typical Trochosphere, the first sign of segmentation appears in the mesoderm bands. These arise by division of the primary mesoderm cells of the embryo, which form two colmnns of cells extending downwards. At the same time the posterior region of the larva elongates greatly, carrying the larva hind-gnt with it. Externally the growing body is marked by constrictions, internally by mesoderm cavities. These cavities, taken together, form the adult body cavity ; the outer and inner walls form the somatic and splanchnic layers ; the posterior and anterior walls of adjacent segments fuse to form the septa of the adult worm ; the inner (splanchnic) walls of the primitive segments on each side fuse above and below the gut to form the dorsal and ventral supporting mesenteries of the gut. The head region is at first disproportionately large, but later, by an independent process of growth, becomes reduced. The larva abandons its pelagic life, and becomes adult. Comparing the development of Polychasta with this, we find that the Trochosphere is often modified, and that segmentation tends constantly to appear at an earlier stage. As a further step in the same direction, we may note that in some Polych»ta the Trochosphere stage is no longer recognisable as such. The two chief orders of this class may be contrasted : — Oligoch^ta, e.g. Earthworm. PoLYCHETA, e.g. Nereis. With no parapodia, and with relatively few setae. Without any " jaw " apparatus in the pharynx. Head not highly developed. No tent- acles or cirri. Gills in a few forms. With complex hermaphrodite reproduc- tive organs, limited in number and definitely localised. Development direct. Living in fresh water or in the soil. With parapodia and with very numerous setae. The pharynx is often armed with "jaws." The head is much more developed, and bears tentacles and cirri. Gills are often present. Sexes usually separate, and reproduc- tive oi'gans simple. A metamorphosis in development. Marine, with two or three exceptions. General Survey of the Class Ch^topoda I. Oligochaeta. — The general characters may be gathered from the description of the earthworm, but it is to be noticed that the earth- worms are specialised forms, and that the fresh-water Oligochaetes are of much simpler structure. The most essential distinction from the Polychaeta is to be found in the complex reproductive organs. The absence of gills, though general, is not universal, for a few fresh-water forms, such as Dero and Branchiura, possess gills of simple structure, while the West African Alma niloiica has more complex liranched retractile gills. Among other characters may be noticed the tendency Fig. 143. — Development of Polygordius. — After Fraipont. a, Mother sperm cell ; b, c, sperm inorulae ; d, spermatozoa. I, Ovum with large nucleus ; 2, two-cell stage ; 3, four-cell stage ; 4, blastosphere ; 5, gastrula ; ac, archenteron ; 6, closure of gastrula mouth or blastopore ; 7, formation of stomodaeum (st.), and proctodaeum {pr.), which invaginate to meet archenteron (ac.) ; 8, complete gut formed ; 9, elongation of larva ; ap. sp., apical spot ; cil., cihated ring ; neph., primitive nephridia ; 10, formation of posterior segments ; 11, form of adult Polygordius. 266 PHYLUM ANNELIDA to variation in the structure of the excretory system. In all, with the exception of Molosoma, certain of the nephridia are modified to serve as genital ducts, while in the Megascolicid^e the nephridia tend to be reduced to a mass of minute tubules ramifying over the inner surface of the body wall. In general the Ohgocha^ta, however, show more uniformity of structure than their marine alhes. They may be divided into two main groups — (i) the Microdrih, and (2) the MegadriU. The first group includes the small aquatic forms ; of these most familiar are Tuhifex rivulorum, often found in the mud of brooks, and the species of Nais, remarkable for their power of asexual budding. Some Microdrili live between tide-marks. The leech-like Branchiobdella, which is parasitic on the gills of the fresh-water crayfish, is a somewhat aberrant member of the group. The MegadriU include the larger Oligochaeta, mostly living in earth, and commonly designated as " earthworms." The largest form is a Tasmanian species {Megascolides gippslandicus), measuring about 6 ft. in length. II. Polychaeta. — As contrasted with the more or less subterranean earth- and mud-worms, the marine Polychaeta have a richer develop- ment of external structures and a more complex life-history. The external appearance is greatly modified by the relative degree of development of the parapodia, which are lateral outgrowths typically functioning as walking " legs," or as swimming organs. A para- podium, when fully developed, is divisible into a ventral neuropodium and a dorsal notopodium. Each of these is bilobed, bears a tactile process or cirrus, and is fringed with firm bristles or seta^. Within the substance of each lobe is embedded a stout needle-shaped " aciculum," which functions as an internal skeleton, both by giving support and by serving as an attachment for muscles. With the notopodium, further, true gills containing prolongations of the body cavity are often associ- ated. Such typical parapodia occur especially in the active free-living forms like Nereis and its allies, but in the order in general the parapodia show much variation, and may be almost suppressed, as in Arenicola. Parapodia are absent from the " prostomium," and are rarely fully developed on the first true segment or peristomium. In both cases, however, tactile cirri and tentacles are often present. The prostomium varies greatly in development and structure, and is of great systematic importance ; it is frequently furnished with eyes and other sense organs, but these may also occur on other regions of the body. Apart from the parapodia, the shape and appearance of the body are most affected by the condition of the septa. In the active free-living forms (Errantia) these are usually present throughout the body, and give a characteristic worm-like appearance. In burrowing and tubicolous forms (Sedentaria) the septa tend to be suppressed. Their absence facilitates burrowing, by permitting free movement of the ccelomic fluid, and is often associ- ated with a division of the body into regions, and a loss of the typical uniform shape (cf. Arenicola). With regard to internal organs, the gut is frequently branched and of large calibre. In some cases (Capitellidas) it possesses an accessory communicating tube (Nebendarm), which is of interest, because it has been compared to the notochord of Vertebrates. There is typically a " WORM-FEET " OR PARAPODIA 267 pair of nephridia in each segment, but they are often reduced in number. They may open into the coelom by a ciliated funnel or nephrostomy or end in a group of solenocytes, which are comparable to the flame-cells of Flatworms (see Fig. 114, A). With the nephridia there are often associated ciliated " coelomoducts," which typically open to the exterior and into the coelom. They often combine with the nephridia. Though the sexes are usually separate, there are a few hermaphrodite forms. There is a metamorphosis in development, and some interesting peculiarities occur in regard to reproduction. Thus several species of the common genus Nereis, when sexually mature, have the body divided into two regions — a posterior region Fig. 144. — Parapodium of " Heteronereis Nereis pelagica. — After Ehlers. of I, 2, 3, 4, The leaf-like outgrowths ; ci., notopodial cirrus ; c~., neuropodial cirrus ; fli., a'-., acicula or supporting bristles of notopodium and neuropodium ; s., setce. containing the ova or sperms, and an anterior unmodified asexual region. The posterior region is distinguished by the structure of its parapodia, which become converted into broad, flattened swimming organs, and there is sexual dimorphism. Worms of this peculiar type were long described as a distinct genus under the name of " Heteronereis," and even yet the subject is imperfectly understood, for there is from unknown causes much variation as regards the extent of the modification. A complete change of habit at the spawn- ing season is probably common here as elsewhere in marine Inverte- brates. In the Syllidce a phenomenon occurs similar to the formation of a " Heteronereis," but a process of fission may result in the division of the modified form into an anterior asexual zooid and a posterior sexual 268 PHYLUM ANNELIDA one. In Mynanida a long chain of sexual zooids is formed. In this way a regular alternation of sexual and asexual generations may arise. Some Polycha)ta dwell habitually within tubes, others are at least at times active and free-living. The latter have usually well-developed parapodia and sense organs, the anterior part of the gut may be furnished with strong jaws, the body is more or less uniform, and the worms are carnivorous. These forms are all included in the sub-order Nereidiformia, which embraces such familiar animals as the common sea-mouse (Aphrodite), with its mass of iridescent bristles covering the sides of the body, the species of Nereis and Ncphthys, so common under stones on the shore, and others equally remarkable for beauty of Fig. 145. — Free-living Polychaete {Nereis cultrifera). Note, as compared with Arenicola, the absence of gills, and tlio vvcll- developed parapodia, which are absent from the peristomium (pe.), or first true segment. The prostomimn bears eyes {e.), the small tentacles (t.), and the large palps (p.) ; c, the four paired cirri, borne by the peristomium ; a., the anus, with two long cirri. colour. The bright colours may be due to the iridescent cuticle or to pigments. There are a few transparent pelagic forms, e.g. Tomopteris. The sedentary forms lead a sluggish life within various kinds of tubes — limy, sandy, papery, or gelatinous. They are not nearly related, but possess in common certain adaptive characters, such as the aggregation of gills, cirri, tentacles, and sense organs to the anterior exposed part of the body ; the reduction of the parapodia, often used solely for clambering in the tube ; the absence of " jaws," and the habit of feeding on minute Algae or other organisms suspended m water. Among these are included Serpula, which forms twisted limy tubes outside shells and other marine objects ; the aberrant Sabellaria. which often builds reefs of porous rock formed of the aggregated sandy SEX DIMORPHISM 269 B.L. PR. tubes ; the common Terebella or Lanice conchilega, with its tubes of glued sand particles ; and the strange phosphorescent Chceiopterus, found in deep water, within its yellow parchment-like tube. III. Echiuridae.— In holes in the rocks on the warmer coasts of Europe there lives a curious " worm " — Bonellia viridis, of a beautiful green colour, with a globular body, and a long, grooved, anteriorly forked, pre-oral protrusion. Such, at least, is the female ; but the male is microscopic in size, lives in or on his mate, and is exceedingly degenerate. His gut is without mouth and anus, the surface is covered with cilia, and the body cavity almost obliterated. Related to Bonellia, but of less anomalous shape, are a few other forms, like Thalassema and Echiurus. In all, the body in the adult shows mere traces of segmentation ; parapodia, cirri, a'Vid gills are absent, but, except in the degenerate males, a few setae are always present. The most characteristic structure is the elongated sohd proboscis, which has the mouth at its base. The nervous system consists of a gullet -ring and a ventral cord, but the latter is unsegmented, and there is no brain. The gut is coiled, and bears a curious adjacent tube known as the " col- lateral intestine," and a pair of excretory . " anal vesicles," opening from gut to body cavity, and formed in development from MALE nephridia. The anus is terminal, there is a closed vascular system, and one to four pairs of nephridia. The sexes are separate, the reproductive elements are formed on the walls of the body cavity, and are shed into it. There is a metamorphosis in develop- ment, but the natiire of the larva differs markedly in the different genera. In Mo. ..io Lo'oj F Fig. 146. — Sex dimorphism in Bonellia viridis, about natural size. The female (F.) has a body about 2 inches long, bearing a pre-oral, ventrally-grooved proboscis {PR.), often 6 inches or more in length when fully extended. The proboscis ends in two arms (B.L.), often recurved. The ciliated groove of the food- catching proboscis leads into the mouth (Mo.). The degenerate male (M.) is about a sixteenth of an inch in length and lives as a parasite, first on'the proboscis, then in the mouth, and finally in^the nephridium of the female. ■''T' ■so, la=^ fl i?'0 ,Co<5^ '^^ 270 PHYLUM ANNELIDA Echiurus and Thalassema it bears a striking resemblance to a Trocho- sphere. Thus there is a well-developed pre-oral lobe with an apical sense organ, and pre-oral and post -oral bands of cilia. " Head kidneys " or provisional nephridia occur, and the post -oral region shows distinct segmentation, the segments being marked externally by rings of cilia. As development proceeds, all trace of segmentation is lost. In Bonellia the larva shows no trace of segmentation, and is Turbellarian-like ; owing to a premature arrest of development, the male remains at this level throughout life. Appendix (i) to Chcetopoda Primitive Forms. Archi-Ch^topoda or Archi-Annelida There are a few small, simple marine worms, with some Annelid or Chaetopod characters, which are sometimes supposed to be ancestral forms. Thus DinophiUis is a minute Planarian-like animal found among Algae. In the young at least the body is distinctly segmented, but there are no bristles, gills, or tentacles. There are circling band* of cilia. The nervous system consists of a brain and two widely separated ventral ganglionated cords, but it remains in contact with the epidermis. More distinctly Annelid are the marine worms Polygordius, Proto- drilus, Saccocirrus, and Histriodrilus. The small body is segmented and uniform ; there are no setae, parapodia, cutI, or gills, but the head bears a few tentacles ; the pre- oral region is small, and the segment around the mouth is large ; the very simple nervous system is retained in the epidermis. Polygordius (Fig. 143 (11)) is a thin worm, an inch or more in length, living at slight depths in sand or fine gravel, often along with the lancelet. It has two tentacles, a few external ciUa about the mouth in a pair of head-pits, and sometimes on the body ; it moves like a worm, but has no bristles. It feeds like an earthworm, or sometimes more discriminatingly on unicellular organisms. The females are usually larger than the males, and in some species break up at sexual maturity. The development includes a metamorphosis, and the larvae are typical Trojchospheres, ciliated, free-swimming, light -loving, surface animals, feeding on minute pelagic organisms, seeking the depths as age advances. Protodrilus is even smaller than Polygordius, with more cilia, mobile tentacles, and two fixing lobes on the posterior extremity ; the move- ments are Turbellarian-like, the reproductive organs hermaphrodite, the development direct. Histriodrilus is parasitic on the eggs of the lobster, and its affinities are doubtful. Appendix (2) to Chcetopoda Parasitic and Degenerate Chaetopods. Myzostomata The remarkable forms {Myzostorna) included in this small class, live parasitically on feather-stars, on which they form galls. They are MYZOSTOMATA 27 1 regarded as divergent offshoots from primitive Annelids, the larval form showing some distinctly Cha?topod characters. The minute disc- like body is unsegmented, and bears five pairs of parapodia, each with a grappling hook, with which four pairs of suckers usually alternate. There are also abundant cirri. The skin is thick, the body muscular, the nervous system is concentrated in a ganglionic mass, which encircles the gullet, and gives off abundant branches. There is a pro- trusible proboscis and a branched gut ; the mouth and anus are ventral. The ova arise in the reduced body cavity, and pass by three meandering oviducts to the anal aperture. The testes are paired, branched, and ventral, with associated ducts, which open anteriorly on the side of the body. The series are united, but there is marked protandry. The very young forms, originally described as " dwarf males," contain sperma- tozoa, and are often carried on the back of the mother ; as they grow older they become hermaphrodite, and later the power of forming spermatozoa is lost and the animals become female. It must be allowed, however, that all would not agree with the above summary. Thus Beard says : " The various kinds of parasitism presented by the numerous species of Myzostoma, have led in some cases to the preservation of the males, in others to their extinction, in yet others to their conversion into hermaphrodites." He distinguishes — 1. Purely dioecious forms with small males, e.g. M. piilvinar. 2. Hermaplirodite forms and true males, which remain males, e.g. M. glabnini. 3. Hermaphrodite forms and males, which, retaining their positions on the hermaphrodites, afterwards become female, e.g. M. alaium. 4. Hermaphrodite forms, in which the males have lost their dorsal position, and have either become extinct or converted into protandric hermaphrodites, e.g. M. cirriferum. Class HiRUDiNEA or Discophora. Leeches This class includes forms in which the body is oval and flattened, usually devoid of setce or gills, and marked ex- ternally by rings which are much more numerous than the true segments. The body cavity is much reduced and broken up {except in Acanthobdella), and may communicate indirectly with the well-developed vascular system. The nephridia are numerous and segmentally arranged. There are usually two suckers, one at each end of the body, the anterior being formed by the mouth. Almost all are hermaphrodite — the male organs are numerous and segmentally arranged, and special genital ducts are present. The genital openings are median. The development is direct. Most live in fresh water or on land, but a few are marine. 272 PHYLUM ANNELIDA Type, the Medicinal Leech {Hiriido medicinalis) Habits. — This is the commonest and most famihar of leeches, once so constantly used in the practice of medicine that leech became synonymous with physician. It lives in ponds and sluggish streams, and though not common in Britain, is abundant on the Continent, where leech farms, formerly of importance, are still to be seen. Leeches feed on the blood of fishes, frogs, and the like, and are still caught in the old fashion on the bare legs of the callous collector. As animals are naturally averse to blood-letting and hard to catch, leeches make the most of their oppor- tunities. They gorge themselves with blood, and digest it slowly for many months, it may be, indeed, for a year. Watched in a glass jar, the leech is seen to move by alter- nately fixing and loosening its oral and posterior suckers, and, on some slight provocation, it will swim about actively and gracefully. At times it casts off from its skin thin transparent shreds of cuticle — a process which, in natural conditions, usually occurs after a heavy meal, when the animal, as if in indigestion, spasmodically contracts its body, or rubs itself on the stems of water-plants. Numerous eggs are laid together in cocoons in the damp earth near the edge of the pool. Thence, after a direct development, the young leeches emerge and make for the water. External features. — The leech is usually from 2 to 6 inches in length, and appears cylindrical or strap-like, according to its state of contraction. The slimy body shows over one hundred skin-rings ; its dorsal surface is beautifully marked with longitudinal pigmented bands, while the ventral surface is mottled irregularly ; the suctorial mouth is readily distinguished from the unperforated hind sucker, above which, on the dorsal surface, the alimentary canal may be seen to end. According to Whitman's precise investigations, there are 102 skin- rings and 26 somites or true segments. The hind sucker is supposed to consist of 7 fused segments, making the total number 33. These segments may be recognised externally by conspicuous pigment spots (" segmental papillae "), which in the middle region of the body occur on every fifth ring. In type, therefore, five rings correspond to a segment, but at either end of the body the number of rings is abbreviated. In the head region there is a pair of " eyes " on the ist, 2nd, 3rd, 5th, and 8th rings : these are homologous with " segmental papillae," and therefore in this region eight rings corre- spond to five segments. STRUCTURE OF LEECH 273 The penis is protruded on the middle ventral line between rings 30 and 31 ; the aperture of the female duct lies five rings farther back. Also on the ventral surface there are seventeen pairs of small lateral apertures, through which a whitish fluid may be squeezed— the openings of the excretory organs. The skin of segments 9-1 1 is especially glandular, and forms the so-called clitellum or saddle, the secretion of which makes the cocoon for the eggs. Skin. — Most externally lies the cuticle — a product of the epidermis — periodically shed, as we have already noticed. In this shedding some of the genuine epidermis cells are also thrown off. These are somewhat hammer-like units, with the heads turned outwards, while the spaces between the thick handles contain pigment and the fine branches of blood vessels. As the latter come very near the surface, a respiratory absorption of oxygen and outward passage of carbon dioxide is readily effected. Opening between the epidermal elements, but really situated much deeper, are numerous long-necked, flask-shaped glandular cells, secret- ing the mucus so abundant on the skin. Underneath the epidermis there is much connective tissue, besides yellow and green, brown and black pigment. Muscular system and body cavity. — The muscular system consists of spindle-shaped cells arranged externally in circular bands like the hoops of a barrel, internally in longitudinal strands like staves. Besides these there are numerous muscle bundles running diagonally through the body, or from dorsal to ventral surface, and there are other muscles associated with the lips, jaws, and pharynx. The body cavity, though distinct in the embryo, is almost obliterated in the adult leech, where the predominant con- nective tissue has filled up nearly every chink. Nervous system and sense organs. — The nervous system mainly consists of a pair of dorsal ganglia lying above the pharynx, and of a double nerve-cord, with twenty-three ganglia, lying along the middle ventral line. The dorsal (or supra-cesophageal) ganglia are connected with the most anterior (or sub-oesophageal) pair on the ventral chain, by a narrow nerve-ring surrounding the beginning of the gut. The sub-oesophageal ganglia represent about five pairs of ganglia fused together. From the dorsal ganglia nerves proceed to the " eyes " and anterior sense spots ; from the ventral centres the general body is innervated. 18 274 PHYLUM ANNELIDA Special nerves from the dorsal ganglia supply the ali- mentary canal, forming what is called a visceral system. The sense organs of the leech are ten so-called " eyes," besides numerous sense spots usually occurring on every fifth skin-ring. The eyes are arranged round the edge of the^mouth,'^and look^like little^blackTspots. Microscopic Fig. 147. — Transverse section, of leech. — After Bourne. c, Cuticle ; e., epidermis ; cm., dermis and outer muscles (circular and oblique) ; l.m., longitudinal muscles (the peculiar connective tissue is hardly indicated) ; r.m., radial muscles ; l.v., lateral blood vessel ; d.s., dorsal sinus ; v.s., ventral sinus enclosing nerve-cord (n.) ; g., median part of crop, with lateral pockets (p.) ; t., testis ; /., nephridial ' funnels ; v.d., vas deferens. examination shows them to be definite cups, surrounded by connective tissue with black pigment, and containing clear, strongly refracting cells, each in connection with a fibre of the optic nerve. It has been shown (Whitman) that the eyes of leeches are serially homologous with the segmental sense organs. At the one extreme there are purely tactile organs, at the other extreme there are purely visual organs, and between these there are compound sense organs. ALIMENTARY CANAL OF LEECH 275 in part tactile and in part visual — a series which is full of suggestive- ness in regard to the evolution of sense organs (of. the series of sensitive setae in the crayfish). The visual organs of the leech are not able to form images of external objects, but the animals are exquisitely sensitive to alterations of light. Alimentary system. — When the leech has firmly fastened itself to its prey by the hind sucker, it brings its muscular mouth into action, pressing the lips tightly on the skin, and protruding three chitinous tooth-plates which lie within . Each of these tooth-plates is worked by muscles, and is like a semicircular saw, for the edge bears from 60 to 100 small teeth. Rapidly these saws cut a triangular wound, whence the flow- ing blood is sucked into the muscular pharynx. The process may be ob- served and felt by allowing a hungry leech to fasten on the arm. As the blood passes down the pharynx, it is influenced by the secretion of glan- dular cells which lie among the muscles of the seventh, eighth, and ninth segments, and exude a ferment which prevents the usual clotting. The blood greedily sucked in gradu- ally fills the next region of the gut — the crop — which bears on each side eleven storing pockets. These become wider and more capacious towards the hind end, the largest terminal pair forming two great sacs on each side of the comparatively narrow posterior part of the gut. As all the pockets point more or less backwards, it is evident that a leech to be emptied of the blood which it has sucked must be pressed from behind forwards. The pockets filled, the leech drops off its victim, seeks to retire into more private life, and digests at leisure. The digestion does not take place in the pockets, but in a small area just above the beginning of Fig. 148. — Alimentary system of leech. — After Moquin-Tandon. m., Mouth ; cr^., sixth crop- pocket ; cyii., last crop- pocket ; v., rectum; s., posterior sucker. 2-76 PHYLUM ANNELIDA the terminal part or rectum. This rectum, running between the two last pockets, is separable from the true stomach just mentioned by a closing or sphincter muscle. It ends in a dorsal anus above the hind sucker. Vascular system. — Two main lateral vessels run longitudinally, one on each side of the body. They are connected with one an- other by looping vessels, give off numerous branches which riddle the spongy body, and have a definite muscular coat. On the dorsal surface and ventrally around the nerve-cord are two lacunar spaces, which are really portions of the true body cavity, and not parts of the vascular system. With those and similar spaces, however, the blood vessels are connected by means of a secondarily de- veloped series of canals, roughly corresponding to the lymphatic vessels of Vertebrates. The blood is red, and contains colour- less floating cells of diverse form. Fig. 149. — Dissection of leech. — After Bourne. e.g., Cerebral ganglia ; p., penis; s.v. is opposite the seminal vesicle ; ov., ovary ; ut., uterus ; v.d., vas de- ferens ; l.b.v., lateral blood vessel ; r.4, fourth testis ; n.v., nephndial vesicle ; N.17, last nephridium ; G.19, nineteenth pair of ganglia; n.c, nerve-cord. I6v n.c NEPHRIDIA OF LEECH 277 Excretory system. — There are seventeen pairs of excretory tubules or nephridia, from the second to the eighteenth segment inclusive. These open laterally on the ventral surface, voiding the waste products extracted from the blood vessels which cover their walls. From the seventh to the seventeenth, each nephridium ends internally in a cihated " cauliflower lobe," corresponding to the funnel of Oligochteta, and enclosed in a blood space, apparently Fig. 150. — A nephridium of leech. — After Bourne. F., Internal terminal funnel ; C, glandular coil covered with blood vessels ; T^., external terminal vesicle. part of the reduced ccelom. In the first nine of these funnel-bearing nephridia the terminal lobe lies close upon and dorsal to a testis, but there is no morphological meaning in this approximation. Each consists of two parts, a twisted horseshoe-shaped glandular region, where the actual excretory function is discharged, and a spherical, internally ciliated bladder opening to the exterior. Within the latter there is a whitish fluid with waste crystals. Reproductive system. — The leech, like many other Invertebrates, is hermaphrodite, containing both male and 278 PHYLUM ANNELIDA female reproductive organs. The essential male organs or testes are diffuse, being represented by nine pairs, lying on each side of the nerve-cord in the middle region of the body. Each is a firm globular body, within which mother sperm cells divide into balls of sperms. The spermatozoa pass from each testis by a short canal leading into a wavy longitudinal vas deferens. This duct, followed towards the head, forms a coil (so-called seminal vesicle) as it approaches the ejaculatory organ or penis. From the coil on each side the sperms pass into a swollen sac at the base of the penis, where, by the viscid secretion of special (" prostate ") glands, they are glued together into packets or spermatophores. These pass up the narrow canal of the muscular penis, and leave the body on the middle ventral line between rings 30 and 31, when they are transferred in copulation to the female duct of another leech. The female organs are more compact. The two small tubular and coiled ovaries are enclosed in spherical vesicles, the walls of which are continued as two oviducts, which unite together in a convoluted common duct. This is surrounded by a mass of glandular cells, which exude a glairy fluid into the duct. Finally, the duct leads into a relatively large muscular sac — the " uterus " — which opens through a sphincter muscle on the middle ventral line between rings 35 and 36. The favourite breeding-time is in spring. Two leeches inseminate one another, uniting in reverse positions, so that the penis of each enters the uterus of the other. Spermatophores are passed from one to the other, and the contained sperms may remain for a long time within the uterus, or, liberated from their packets, may work their way up the female duct, meeting the eggs at some point, or reaching them even in the ovaries. The development is direct. General Notes on Discophora The leeches constitute a relatively small class, whose structure has been insufficiently worked out. The presence of suckers, the parasitic habit, the reduction of the body cavity, have led many naturalists to associate them with Flat-worms, but all recent work goes to emphasise their affinity with Annelids, especially Oligochajtos. In leeches setae LEECHES 279 are absent, except in Acanthobdella, which has paired segmentally- arranged bristles in the anterior region ; but it is to be noted that they are absent in some OHgocha?ta. As in Oligocha^ta, gills are usually absent, but occur in Branchellion. The condition of the body cavity affords one of the most striking contrasts to Oligochaeta ; but in Acanthobdella the adult has a typical Annelid coelom divided into regions by septa. In others, in spite of the large amount of connective tissue in the adult, there are distinct traces of segmental septa. In Hirudo the reduction is carried so far that the coelom is represented merely by canals without trace of septa. In all cases, however, development shows that the reduction is secondary, and that in the embryo there is a true Annehd body cavity unconnected with the vascular system. The condition of the alimentary canal affords a basis for classification, for in one set the anterior region is protrusible, and in the other it is not, but is furnished with jaws or tooth-plates. The jaws are interesting, because they are absent from Oligochaeta, except in a few forms, like Branchiohdella ; the jawed leeches are more specialised than those without these structures. With regard to the nephridia, in Clepsine, which has a fairly well- developed body cavity, there is a direct communication between coelom and nephridia by means of a ciliated funnel of typical Annehd form. Where the coelom is much reduced, as in Hirudo, the funnel is repre- sented by the blind cihated " cauHflower lobe." In the reproductive system, apart from the numerous male organs, the leeches differ from the 01igocha?ta in the apparent continuity of the organs and ducts ; but in the case of the ovaries, at least, the connection is secondary. In the processes of fertilisation and egg-laying, in the formation of a cocoon, and in the development, the two groups show marked resemblance. Most leeches are worm-like aquatic animals, with blood-sucking propensities ; but some live in moist soil, and others keep to the open surface, while the parasitic " vampure " habit, familiarly illustrated by the apothecary's ancient panacea, is in many cases replaced by carnivorous habits and predatory life. The medicinal leech [Hirudo) is typical of the majority, for it lives in ponds and marshes, and sucks the blood of snails, fishes, frogs, or of larger available victims. The giant leech {Macrobdella valdiviana), sometimes measuring i^- ft. when at full length in movement, is subterranean and carnivorous ; while the wiry land-leeches {Hcrniadipsa, etc.), of Ceylon and other parts of the East, move very rapidly along the ground, fasten on to the legs of man or beast, and gorge themselves with blood. The hungry horse- leeches are species of Hcemopis, greedily suctorial, though the teeth, which occur in two rows, are too small and irregular to be useful in medicinal blood-letting ; but the name is also applied to species of the common genus Aulostoma, which are carnivorous in habit. Other common leeches are species of Nephelis, predaceous forms with indis- criminating appetites, and the little Clepsine, also common in our ponds, notable for carrying its young about on its ventral surface. Several marine forms prey upon fishes and other animals, e.g. the " skate-sucker " {Pontobdella muricaia), with a leathery skin rough with knobs. This form lays velvety eggs in empty mollusc shells, and 280 PHYLUM ANNELIDA mounts guard over them for more than a hundred days. The re- markable Branchellion on the Torpedo has eleven pairs of leaf-like respiratory plates on the sides of its body, and so has the related Ozohranchus jantseaniis, a parasite of a river turtle in the Jantsekiang. One of the strangest habitats is that of Lophobdella, on the lips and jaws of the crocodile. Classification. — Family i. Rhynchobdellida, in which the fore part of the pharynx can be protruded as a proboscis. There is an anterior as well as a posterior sucker. The blood plasma is colourless. The ova are large and rich in yolk ; the embryos are hatched at an advanced stage, and soon leave the cocoon, which contains no albuminous fluid. e.g. Clepsine, Pontobdella, Branchellion. Family 2. Gnathobdellidaj, in which there is no proboscis, but the pharynx usually bears three tooth-plates. The mouth is suctorial. The blood plasma is red. The ova are small and without much yolk ; the embryos are hatched at an early stage, and swim about in the nutritive albuminous fluid of the cocoon. e.g. Hirudo, Hctmopis, Hcemadipsa, Aulostoma, Nephelis. Family 3. Acanthobdellidas. By itself is the .Siberian fish parasite Acanthobdella, which has rows of setae on the first five segments, a spacious coelom, and other peculiarities. ASSOCIATED CLASSES The seven classes which follow are associated more or less with Annelid worms, though the relationships are not yet clear. Class Ch^tognatha. Arrow-worms The. pelagic Sagitta and Spadella require a class by themselves. They may be regarded as Annelids with three segments. The trans- lucent body, which may be nearly 3 in. long, but is usually much less, has three distinct regions — a head bearing a ventral mouth with spines and bristles (whence the name Cha^tognatha), a median region with lateral fins, and a trowel-like tail. The nervous system consists of a supra-CESophageal ganglion in the head, a sub- oesophageal about the middle of the body, long commissures between them, and numerous nerves from both ; it retains its primitive connection with the epi- dermis. There are tw'o eyes and various patches of sensitive cells. The food canal is complete and simple, and lies in a spacious ciliated body cavity. Corresponding to the external divisions, the cavities of the head, body, and tail are distinct, being separated from one another by septa ; a longitudinal mesentery supports the gut and divides the cavities into lateral halves. ARROW-WORMS 281 There is no vascular system, nor are there any certain nephridia. It is possible that the latter may be represented by the genital ducts. The animals are hermaphrodite, and the simple reproductive organs lie near one another posteriorly. The two ovaries project into the body cavity, and their ducts open laterally where body and tail meet. The two testes project into the cavity of the tail ; and their ducts have ,<^:S Fig. 151. — Development of Sagitia. — After O. Hertwig. Illustrating formation of a body cavity by pockets from the archenteron ; also the early separation of reproductive cells. Ec, Ectoderm ; En., endoderm ; ac, archenteron ; R., repro- ductive cells ; bl., blastopore ; c.p., coelom pouches ; m., mouth ; i, section of gastrula ; 2 and 3, origin of coelom pouches. internal cihated funnels, and open on the tail. Two reproductive cells are set apart at a very early stage, and each divides into the rudiment of an ovary and of a testis. The eggs undergo complete segmentation ; a gastrula' is formed by the invagination of the blastula ; the body cavity arises, in enterocoelic fashion, as two pockets from the arch- enteron. The young forms are like the adults. Class Rotatoria, Rotifers Rotifers are beautiful minute animals, abundant in fresh water, also found in damp moss, and in the sea. They owe their name and the old-fashioned title of wheel-animalcules to the fact that the rapid move- ments of cilia on their anterior end produce the appearance of a rotating wheel. The food seems to consist of small organisms and particles caught in the whirlpool made by the lashing cilia. The little animals are tenacious of life, and can survive prolonged drought. If they are left dry for long, however, they die, though the ova may survive and subsequently develop. The body is usually microscopic, and is sometimes (e.g. in Melicerta and Floscularia) sheltered within an external tube. There is no internal segmentation, but there are sometimes external rings, and the attaching outgrowth or " foot " is sometimes segmented. The anterior 282 PHYLUM ANNELIDA end bears, on a retractile ridge, the ciliated ring or " trochal apparatus." The nervous system is a single dorsal ganglion with a few nerves. An unpaired eye and some tufts of sensory hairs are usually present. The food canal extends along the body in a well-developed " coelom," and the fore-gut contains a mill, in which two complex hammers beat upon an anvil. The canal ends posteriorly on the dorsal surface between the body and the foot, and, as the terminal portion also receives the excretory canals and the oviduct, it is called a cloaca. There is no vascular system, but a nephridial tube of a primitive type lies on each side of the body, and opens posteriorly into the cloaca. The sexes are separate ; the reproductive organs are simple. Except in the marine parasite Seison, in Rhinops vitrea, and two or three other forms, the males are dwarfed and degenerate, destitute even of a true food canal, and often "little more than perambulating bags of spermatozoa." In many cases the sexual union (effected by a penis) seems to be ineffective, and there is no doubt that many, if not most. Rotifers are parthenogenetic. No males have as yet been found in Philodina, Rotifer, Callidina, or Adineta. The females lay three different kinds of eggs, according to their conditions and constitution — either small ova, which become males, or thin-shelled "summer ova," or thick-shelled " resting or winter ova," the two last developing into females. The so-called winter eggs may occur at any season, and seem usually to have been fertilised. Many species, however, are viviparous. We include the Rotifers beside the Annelids proper, be- cause it seems possible to regard them as derived from ancestors somewhat like Annelid larvae. Rotifers living in fixed tubes or envelopes — Melicerta, Flosciilaria, Stephanoceros. Free Rotifers — Notommata, Hydatina, Brachionus. Parasitic on the marine crustacean Nehalia — Seison. Pedalion occupies a unique position ; it has hints of appendages and a peculiar jumping motion. A. Class SiPUNCULiD^, e.g. Sipiinciilus , and B. Class Priapulid^, e.g. Priapidus These two classes were formerly united with the Echiuridce as Gephyrea, but it is improbable that the three are nearly related. The Echiuridce are apparently modified Cha^topods, while the position of the Sipunculidae and Priapulidae is quite uncertain. Both include marine worms, living in the sand or mud upon which they feed, having unsegmented bodies with a capacious body cavity, and an anterior protrusible proboscis or introvert, which is moved by special retractor muscles, and bears the mouth at its tip. In most other respects the two classes differ markedly from one another. In the Sipuncuhds, the large introvert terminates in a hollow tentacular fringe, within the cavity of which closed blood vessels run. The gut is much coiled, and the anus is dorsal and anterior. A nervous Fig. 152. — Diagram of the structure of a Rotifer [Hydatina). — After Plate. 1. Attaching foot. 2. Cement glands. 3. Bladder into which renal tubes open. It communicates with the exterior via the cloaca (14). 4, 5. Ova lying in the oviduct. 6. A longitudinal muscle. 7. Gastric gland. 8. Gizzard or mastax, with masticating apparatus. 9. Ciliated lobes at the anterior end. A ciliated funnel leads down to the mouth. 10. Gullet. 11. A muscle. 12. Large cells forming the wall of the stomach. 13. A renal tube, also seen anteriorly. 14. The cloaca. 284 PHYLUM ANNELIDA system with a distinct brain, a gullet-ring, and a ventral cord is present. but the ventral cord is unsegmented. Peculiar ciliated vesicles or " urns " arise in some Sipunculids as buds from the blood vessels, and many swim freely in the body cavity. By collecting and agglutinat- ing particles they help to purify the ca?lomic fluid. Large nephridia or brown tubes, usually two in number, occur in the anterior region, and function also as genital ducts. The sexes are separate except in Phascolosoma minutum, and the reproductive cells develop on the lining of the body cavity. In the development, which includes a meta- morphosis, several peculiarities are observable, tending to show that the animals are not primitive. The larva of Sipunculus is sometimes compared to a trochosphere, but differs from a typical trochosphere, notably in the total absence of segmentation, of " head kidneys," of a pre-oral band of cilia, as well as in the position of mouth and anus, and the slight development of the pre-oral lobe. The class includes eleven genera, which are widely distributed ; many of the species are large and conspicuous. It should be noticed that while Sipunculids are typically without trace of setae, some genera, e.g. Phascolosoma, have distinct hooks on the introvert. The PriapulidEe include two genera — Priapulus and Halicryptus, both almost entirely confined to the northern hemisphere. They have no tentacles, no vascular system, no brown tubes, and no brain. The gut is straight, or has a single loop ; the anus is posterior. A gullet -ring and ventral nerve-cord are present as in Sipunculus, but retain their primitive connection with the epidermis. There are complex genital ducts opening by a pore on each side of the anus, which in the young are connected with an excretory system of the Platyhelminth type, while in the adult they are overgrown and concealed by the repro- ductive cells. The development is unknown. In Priapulus there is a peculiar respiratory (?) appendage at the posterior end of the body. MOLLUSCOIDEA The three classes — Phoronoidea, Polyzoa or Bryozoa, and Brachio- poda are sometimes grouped under the old, not very happy, term RIOLLUSCOIDEA. The Molluscoidea are characterised by the presence of a true coelom, formed in development by the folding off of pouches from the archenteron, and by the shortening of the dorsal region of the body, which results in the close approximation of mouth and anus. The mouth is typically furnished with ciliated tentacles, and is often overhung by an epistome ; both tentacles and epistome, when present, contain spaces which are part of the body cavity. Except in the Ectoprocta, among Polyzoa, two or four nephridia are present, and serve also as genital ducts. There is always a metamorphosis in development, and the larva^ are peculiar. The development is in most cases insufficiently known, and it is probable that further knowledge of it will remove these sets of animals from their apparently anomalous position. SO-CALLED MOLLUSCOIDEA 28s Class Phoronoidea This class was erected for the genus Phoronis, which has been associated both with the Gephyrea and with Polyzoa. Another genus Phoronopsis, from the Cape, has been recently established. It has been proposed to associate these two genera, along with Cephalodiscus CNR Fig. 153. — Actinotrocha or larva of Phoronis. — After Masterman. The mouth is overhung by the prominent pre-oral hood ; the anus is at the other end of the body. Behind the mouth is a ring of ciliated tentacles. SP., the nerve ganglion in the hood ; N.G., the nerve gan- glion of the region called collar region by Masterman ; CNR., nerve-ring at base of tentacles. Fig. 154. — Phoronis, much enlarged. TR., Trunk ; TE., tentacles ; TU., tube. and Rhabdo pleura, with the Hemichorda, on account of certain Chordate affinities said to be exhibited by the larva. But the evidence for this is very unconvincing. The genus Phoronis includes a few species of small marine " worms," social in habit, and found enclosed in fixed leathery tubes often en- crusted with foreign particles. Each individual is furnished with a horseshoe-shaped crown of tentacles, which are hollow and supported 286 PHYLUM ANNELIDA by an internal skeleton. The nervous system lies in the ectoderm — a very primitive character, and consists of a ring round the mouth, and of a cord down the left side of the body. An interesting point is the presence of a closed vascular system with nucleated red cells. The body cavity is well developed, and is divided into chambers. The sexes are united ; and the larva, known as Actinotrocha, undergoes a remarkable metamorphosis in the course of its conversion into the adult. Class PoLYZOA or Bryozoa As usually defined, the class includes two sub-classes, the Ectoprocta and the Entoprocta, but it seems almost certain that these are distinct classes. The Ectoprocta include fresh-water and marine forms, in which the Fig. 155. — Diagram of an Ectoproctous Polyzoon {PUimatella). L., Lophophore ; PH., pharynx ; A., anus ; S., stomach ; M., retractor muscle ; F., funiculus, a cord of mesodermic tissue ; O., cells that form " statoblast " buds ; B., an ordinary bud ; £., epistome over the mouth ; T., ten- tacles ; S/f., outer wall of zooecium. anus is outside the bases of the tentacles. The nervous system is repre- sented by a ganglion placed between the mouth and anus. There is a body cavity. There is no vascular system. Nephridia are absent. All are colonial and bud very freely ; the marine forms show considerable division of labour among the members of the colony. LAMP-SHELLS 287 (a) Tentacles in a crescent — Fresh water, Cristatella, Lophopus, etc. {h) Tentacles in a circle — Marine, except Paludicella ; Flustra, the common sea-mat ; Memhranipora, encrusting seaweed, etc. ; Cellepora, very calcareous ; Alcyonidium, gelatinous. The Entoprocta include the colonial Pedicellina, with a few allied genera, also the non-colonial Loxosoma, in which the buds separate as soon as they are formed. All the forms are stalked and minute. The anus is included within the tentacular circle. In the meta- morphosis of Pedicellina there is an elongation of the dorsal region of the body, and a consequent approximation of the mouth and anus on the shortened ventral surface. There is no apparent body cavity in the adult, and the mesoderm arises from two primitive mesoblasts. The nephridia are anterior, minute, and do not serve as genital ducts, but resemble the protonephridia of Annelid trochospheres. They are said to terminate in flame-cells like those of Platyhelminthes. In all these three respects the Entoprocta differ from the Ectoprocta, and from the Molluscoidea generally. Class Brachiopoda The Brachiopods or Lamp-shells are quaint marine animals, once very numerous, but now decadent. The body is enveloped dorsally and ventrally by two folds of skin or mantle ; these secrete a shell, usually of lime, but sometimes organic. The development of this shell has apparently modified both the position and the relations of the organs. There is no real resemblance between a Brachi- opod shell and that of a bivalve Mollusc, except that both consist of two valves. In Brachiopods these lie dorsally and ventrally ; in Lamellibranchs they are lateral ; moreover, in Brachiopods the ventral valve is usually the larger. It is hardly necessary to say that the Brachiopod organism is not the least like a Mollusc. A considerable part of the space between the valves of the shell is filled up by two long " arms," which are coiled in a spiral, and often supported Fig. 156. — Interior of by a calcareous skeleton. These arise Brachiopod shell, showing in development from the specialisation calcareous support for the of a horseshoe-shaped " lophophore," "arms."— After Davidson, such as is characteristic of the Polyzoa. The mouth is placed between the arms, and opens into the ciliated food canal. This may end blindly, or may be furnished with an anus placed near the mouth ; in Crania the anus is dorsal and posterior. The muscular system is well developed, the shell being both opened and closed by means of muscles. There is a nerve- ring round the gullet, with a slight brain and an inferior gang- 288 PHYLUM ANNELIDA lion. Sensory structures in many cases perforate the valves. Above the gut lies the heart, which is connected with blood vessels. Two (or more rarely four) nephridia open near the mouth, and serve also as genital ducts. The posterior region of the body often forms a stalk by which the shell is moored, but in many this stalk is absent, and the animal is directly attached to the substratum. The sexes are sometimes separate, but perhaps some are hermaphrodite. There is a metamorphosis in the development, and the larva? resemble, in some respects, those of Polyzoa. The Brachiopods date from the earliest known fossiliferous rocks, and had their maximum representation in the Ordovician and Silurian. CHAPTER XII PHYLUM ECHINODERMA Class I. HoLOTHUROiDEA. Sea-cucumbers. 2. EcHiNOiDEA. Sea-urchins. | Sub-Phylum 3. AsTEROiDEA. Starfishes. TEleutherozoa. ■] 4. Ophiuroidea. Brittle-stars. 5. Crinoidea. Feather-stars. ^ 6. Edrioasteroidea. Extinct. I Sub-Phylum 7. Blastoidea. Extinct. | Pelmatozoa. 8. Cystidea. Extinct. ) In contrast to the worms, the Echinoderms form a well- defined series. They may be described as sluggish marine animals, generally with superficially radial symmetry, with a tendency to form limy skeletons. The radial symmetry led the older zoologists to place the Echinoderma near Coelen- tera, but there seems to be no real affinity. Moreover, the larval Echinoderm is bilateral in its symmetry. It seems likely that the Echinoderms represent an ofi"shoot of some *' worm " stock. As in Coelentera, the nervous system shows a marked absence of centralisation, which may be connected with the absence of a definite head region, and this again with the sedentary or sluggish habit. General Characters The Echinoderms are coelomate marine animals in which the bilateral symmetry of the larva is replaced in the adult by more or less marked radial symmetry. In addition to the dominant radial symmetry, the adults show to a varying extent a tendency towards the bilateral type, but this is never the same as that of the larva, nor is it equivalent in the different forms. Lime is always deposited in the mesodermic tissues {mesenchyme), and in consequence there is frequently a 19 290 PHYLUM ECHINODERMA very complete skeleton. From the primitive gut of the larva, pouches grow out to form the usually spacious ccelom and the characteristic water vascular syste?n {hydrocoel), which may have locomotor or respiratory functions or both. The branches of this system, together with the nerves, exhibit in most cases a typical five-rayed arrangement. In addition to the water vascular system, there is an ill-defined lacunar system of blood vessels. In the hcemal vessels, water vessels, and coelom, there are abundant migratory amoebocytes. Well-defined excretory organs are absent. Gonads arise on the lining of the body cavity, and are radi- ately disposed except in Holothurians . The sexes are almost always separate. There is usually a striking circuitousness or indirectness in development. The larvce are almost always free- swimming, and exhibit a metamorphosis. The diet is vegetarian {most sea-urchins), or carnivorous {starfishes), or consists of the organic particles found in sand and mud, the Holothurians in particular practising this worm-like mode of nutrition. Most Echinoderms have to a remarkable extent the power of casting off and regenerating portions of their body. This power is probably one of their means of defence, but they often mutilate themselves as a consequence of unfavourable condi- tions of life. This self -mutilation, or autotomy, seems to be reflex, and not voluntary. '"'>.. -Pluteus larva of Ophiur- oid, with rudiment of adult. — After J ohannes Miiller. Fig. 157. General Notes on Structure The Echinoderma, in spite of the numerous fossil representatives, form an exceedingly well-defined group, showing no close relation to any other, and exhibiting certain striking peculiarities. The skeleton is generally well developed ; in Holothurians it consists of isolated ECHINODERM CHARACTERS 29 1 spicules, but elsewhere of a series of plates which may be firmly united together, as in most sea-urchins, or may be capable of movement upon one another. Apart from the skeleton proper, lime may appear in almost any of the organs of the body. With this deep-seated tendency to form skeletal substance may perhaps be associated the sluggish habit of the majority, and the absence of definite excretory organs. Except in Holothurians, where the calcareous plates are diffusely scattered, the parts of the skeleton show much regularity of arrangement. The primitive skeleton is believed to have consisted of two series of plates, constituting respectively the oral and apical systems. These, especially the latter, were of much importance in the formation of the skeleton of the extinct Blastoids and Cystoids, but in modern Echinoderms they are absent or unimportant, and are functionally replaced by accessory plates, such as those which form the " test " of sea-urchins. The oral system consists of five plates surrounding the mouth, and in living forms it is fully developed only among Crinoids. The apical system in the Pelmatozoa typically forms a cup or calyx enclosing the viscera, and consists of a central plate to which a stalk may be attached, and three sets of plates arranged around this, five infra-basals, five basals, and five radials. In the larva of Antedon this apical system is fully represented, except that the infra-basals are reduced to three, but in other Crinoids and in the adult Antedon there tends to be reduction. Among other Echinoderms the apical system is best represented among sea-urchins, where there are often five basals (the genitals) around the anus. The " oculars " seem to correspond to the " terminals " at the tips of starfish arms. In Ophiuroids the apical system is sometimes repre- sented both by basal and radial plates, but often only by radials ; in starfishes it is typically absent in the adult, though more or less clearly shown in the larva. The other most striking characteristic of Echinoderms is the peculiar water vascular system. This arises in development from the coelom, and consists typically of the following parts : — An external opening or madreporite opens into a canal with calcified walls, called the stone canal ; this opens into a ring canal around the mouth, which has often connected with it little vesicles and glandular bodies ; the ring canal opens into five radial canals which run in the radii of the body, and give off branches to the protrusible tube-feet which project on the surface of the body, and may be furnished with suckers ; the radial canals are also often connected with internal reservoirs or ampullae. The tube-feet are very characteristic, and have different functions in the different classes. In Asteroids, in most Holothurians, and in part in Echinoids, they are primarily locomotor ; in Ophiuroids, in Crinoids, and in part in Echinoids, they are respiratory, tactile, or used for food- catching. But there is great variety of structure and functions ; thus in many Holothurians the tube-feet are represented only by a ring of tentacles around the mouth. 2g2 PHYLUM ECHINODERMA Class AsTEROiDEA. Starfishes Star-like or pentagonal Echinoderms more or less flattened at right angles to the main axis of the body ; usually with well-defined simple arms containing the gonads and prolonga- tions of the gut, and with a ventral ambulacral groove sup- ported by paired ossicles and bearing the tube-feet ; with regularly disposed calcareous, often spinous, plates on the skin ; with an external madreporite {occasionally multiple), always on the upper surface of the disc in living forms ; with a mouth at the centre of the lower surface, and usually with an anus at the opposite pole. Description of a Starfish The description apphes especially to the common five- rayed starfish {Asterias or Aster acanthion rubens). It is often seen in shore pools exposed at low water, but its haunts are on the floor of the sea at greater depths. There it moves about sluggishly by means of its tube-feet. Each of the five arms bears a deep ventral groove in which the tube-feet are lodged. The mouth is in the middle of the ventral surface, the food canal ends about the centre of the dorsal disc. With this flat, five-rayed form, the 11-13 rayed sun-star {Solaster), the pincushion- like Porania, and the flat pentagonal Palmipes, should be contrasted. Between two of the arms lies the perforated madreporic plate, thus defining the bivium, while the three other arms constitute the trivium. The body is covered by a ciliated ectoderm, beneath which lies a mesodermic layer. In association with the latter there is developed on the ventral surface of each arm a double series of sloping plates. These meet dorsally, like rafters, in the middle line of the arm, forming an elongated shed. The rafter-like plates are called ambulacral ossicles ; the groove which they bound lodges the nerve-cord, the water- vessel, and the tube-feet of each arm. In association with the outer mesodermic layer of the integument, numerous smaller plates are developed, e.g. the adambulacrals, which articulate with the outer lower ends of ambulacrals. The dorsal surface bears a network of little ossicles, and many of these bear spines. Peculiarly modi- fied spines, known as pedicellarice, look like snapping STRUCTURE OF STARFISH 293 scissor-blades mounted on a single soft handle. They have been seen gripping Algae and the like, and probably keep the surface of the starfish clean. A starfish is not very muscular, but it often bends its arms upwards by means of a muscular layer in the body wall. Other muscles affect the size of the ventral grooves, and muscular elements also occur on the protrusible part of the stomach, and in connection with the water vascular system. Fig. 158. — Starfish. I. Ventral surface ; t.f., tube-feet extended ; a.g., the ambulacral groove with the tube-feet retracted ; m., the mouth. II. Dorsal surface, showing the position of the madreporite (M.) ; the two adjacent arms form the bivium. Underneath the ciliated ectoderm lies a network of nerve fibres, with some ganglion cells. But besides these diffuse elements there is a pentagon around the mouth, and a nerve along each arm. The system is not separable from the skin. Ganglion cells are developed also on certain parts of the wall of the coelom. A red eye spot, sensitive to light, lies on the terminal ossicle at the tip of each arm, and is usually upturned. It is a modified tube-foot, bearing numerous little cups, lined 294 PHYLUM ECHINODERMA by sensitive and pigmented cells, containing clear fluid, and covered by cuticle. The skin is diflFusely sensitive. The terminal tube-foot of each ray seems to be olfactory. Fig. 159. — Dissection of Asteyias rubens. — From a specimen. ST., Roof of central stomach ; A.G., two anal glands ; P.C, pyloric caeca, five pairs ; A.O., ainbulacral ossicles, meeting like rafters ; A., ampullae ; S.C., stone canal from the madreporic plate to the circumoral water-vessel ; S.St., saccular dilatations of the stomach ; G., gonads, a pair in each arm. The starfish may be found with part of its stomach extruded over young oysters and other bivalves. This protrusible portion of the stomach is glandular and saccu- STRUCTURE OF STARFISH 295 lated, and bulges slightly towards the arms ; it is followed by an upper portion, giving off five branches, each of which divides into two large digestive caeca — a pair in each arm (Fig. 159). These glands are comparable to a pancreas ; their secretion contains three ferments, which convert proteins into peptones, starch into sugar, and break up fats into fatty acids and glycerine. From the short tubular intestine between the stomach and the almost central dorsal anus two little outgrowths are given off, perhaps homologous with the " respiratory trees " of Holo- thurians (Fig. 167, r.t.). Some parts of the food canal are ciliated. The ccelom is distinct, though not much of it is left unoccupied either in the disc or in the arms. It is lined by ciliated epithelium, and contains a fluid with amoeboid cells. A few of these have a pigment which probably aids in respiration ; others are phagocytes, which get rid of injurious particles through the *' skin-gills " ; others con- tinue the work of digestion. When a starfish is crawUng up the side of a rock, scores of tube-feet are protruded from the ventral groove of each arm ; these become long and tense, and their sucker-like terminal discs are pressed against the hard surface. There they are fixed, and pull up the starfish by muscular contrac- tion. The protrusion is effected by the internal injection of fluid into the tube-feet ; the fixing is due to the pro- duction of a vacuum between the ends of the tube-feet and the rock. As to the course of the fluid, it is convenient to begin with the madreporic plate, which Hes between the bases of two of the arms (the bivium). This plate is a complex calcareous sieve, with numerous perforating canals and external pores. It may be compared to the rose of a watering-can, but the holes are much more numerous, and lead into small canals, which converge into a onain ciliated canal, the stone canal. This, as usual, opens into a ring canal around the mouth. The ring canal gives off nine glandular bodies (Tiedemann's bodies), and five radial tubes, one for each of the arms. Considerations of symmetry suggest that there should be ten glandular bodies, but in the inter-radius containing the stone canal there is only one. In many starfishes there are five or ten little reservoirs (Polian vesicles) opening into the circumoral ring, but in Asterias rubens these are hardly dis- tinguishable from the first ampullae of the radial vessels. These run along the arms, and lie in the ambulacral groove beneath the shelter of the rafter-like ossicles. From them branches are given off to the 296 PHYLUM ECHINODERMA bases of the tube-feet, but from each of these bases a canal ascends between each pair of ambulacral ossicles, and expands into an ampulla or reservoir on the dorsal or more internal side (see Fig. 160). The fluid in the system may pass from the radial vessels into the tube-feet, and from the tube-feet it can flow back, not into the radial vessel, but into the ampullae. There are muscles on the walls of the tube-feet, ampullae, and vessels. At the end of each arm there is a long unpaired tube-foot, which seems to act as a tactile tentacle, and has also olfactory significance. With regard to the vascular system there is considerable uncertainty. Fig. 160. — Diagrammatic cross-section of starfish arm. — After Ludwig. «., Radial nerve ; b.v., radial blood vessel according to Ludwig, septum in pseud-hasmal vessel according to others ; w.v., radial water vessel ; am., ampulla ; t.f., tube-foot ; p.c, a pyloric caecum cut across ; s.p., a calcareous spine ; g., a skin- gUl ; lac, spaces in the wall ; go., ova in ovary ; a.o.. ambu- lacral ossicle. There is probably no definite vascular system at all. The organ described as a heart is really the " genital stolon." There is a " pseudhaemal sinus " surrounding the stone canal, leading into a circumoesophageal ring, which gives off a vessel along each ray. From the dorsal surface and sides of a starfish in a pool, numerous transparent processes may be seen hanging out into the water. They are the simplest possible respiratory structures, contractile outgrowths of the skin with cavities continuous with the coelom, and are called " skin-gills," STRUCTURE OF STARFISH 297 It is likely that pigmented cells of the body cavity fluid act as rudimentary red blood corpuscles ; the water vascular system may help in aeration ; and the whole body is, of course, continually washed with water. The " skin-gills " are said to have an excretory function ; for phagocytes, bearing waste, seem to traverse their walls. It may also be that excretion is somehow concerned in forming the carbonate of lime skeleton, but facts are wanting. The sexes are separate, and they are like one another, both externally and internally. The gonads develop periodically, and lie in pairs in each arm. Each is branched like an elongated bunch of grapes, and is surrounded by a " blood sinus." Each has a separate duct, which opens on a porous plate, between the bases of the arms on the dorsal sur- face. In Asterina gtbbosa, how- ever, the eggs are extruded ven- trally. In the same species there is an interesting sexual variability : many are first males and then females (protandric), others are simply hermaphrodites, others seem exclusively of one set. The eggs of starfishes are fertilised in the water, and the free-swimming larva is known as a Bipinnaria or as a Brachio- laria. Fig. 161. — Regeneration of a starfish from a separated arm — the so-called " comet form." T.F., Tube-feet in the ambulacral groove [A.G.) of the separated "■ arm, which is growing the other four; M., the mouth; R.A., the radial or ambulacral area of one of the four new arms. Other Starfishes Parental care is incipient among Asteroids. A species of Asterias has been seen sheltering its young within its arms ; 298 PHYLUM ECHINODERMA there is a definite brood-pouch in the form of a sort of tent on the dorsal surface of Pteraster. Many Asteroids break very readily, or throw off their arms when these are seized. The lost parts are slowly regenerated, and strange forms are often found in process of regrowth. Thus the " comet form " of starfish occurs when a separated arm proceeds to grow the other four (Fig. 161). There are many deep-sea forms, such as the ophiuroid- like Brisinga, the widely distributed Hymenaster^ and the blue Porcellenaster coeruleus ; but the majority occur in water of no great depth. Asteroidea first occur in Silurian strata. Classification. — Order I. Phanerozonia. With strongly developed marginal plates, the upper and lower marginals in contact ; with skin- gills restricted to the dorsal (abactinal) surface ; with broad ambulacral plates ; with prominent adambulacrals in the peri- stome, with pedicellariae sessile (if present), with two rows of tube-feet. e.g. Astropecten, Luidia, Porania, Asterina, Palmipes. Order II. Cryptozonia. With indistinct or rudimentary marginal plates in the adults, often with intermediate plates between the upper and lower marginals, with skin-gills not restricted to the dorsal (abactinal) surface, with narrow ambulacral plates, with ambulacrals or adambulacrals prominent in the peristome, with pedicellariffi sessile or stalked (if present), often with apparently four rows of tube-feet. e.g. Aster ias, Solaster, Henricia, Bnsinga. Class Ophiuroidea. Brittle-stars, e.g. Ophiopholis aciileata Echinoderms with a stellate flattened body, nearly related to starfishes, but usually dijfering from them in having the amis {sometimes branched) sharply marked off from the central disc, no ambulacral groove on the ventral surface of the arms, the digestive organs and gonads restricted to the disc, and the madreporite ventral. There is no anus. There are deep respiratory clefts on the disc at the insertion of the arms. They agree with starfishes in being free, in having radially disposed gonads, in having the tube-feet restricted to the under surface, and in other features. The body of a brittle-star diff'ers from that of a star- fish in the abruptness with which the arms spring from BRITTLE-STARS 299 the central disc (cf. Brisinga). These arms are muscular, and useful in wriggling and clambering ; they do not con- tain outgrowths of the gut, nor reproductive organs. Moreover, there is no ambulacral groove, and the tube-feet which project on the sides are usually very small. They are often of locomotor service, adhering even to vertical surfaces, but in many cases they seem to be only sensory. Each segment of the arm includes a central " vertebral Fig. 162. — Ventral surface of disc of an Ophiuroid {Ophiothrix fragilis). — After Gegenbaur. g., Openings of genital pockets or bursae ; m., mouth ; v., ventral plates of arms ; sp., spines of arms ; t.f., tube-feet — at the right side these are represented as retracted ; 0., the openings through which they are protruded ; p., plates around mouth bearing the so-called teeth ; one of these plates is perforated, and functions as the madreporite. ossicle," with four plates forming a tube round about it. There is a complex oral skeleton. -The madreporic plate is situated on the ventral surface, usually on one of the plates around the mouth. The food canal ends blindly. Some brittle-stars have small luminescent glands, e.g. Amphiura squamata. The reproductive organs lie in pairs between the arms, and open into pockets or bursae formed from inturnings of the skin, which communicate with the exterior by slits opening at the bases of the arms. 300 PHYLUM ECHINODERMA Water currents pass in and out of these pockets, which probably have both respiratory and excretory functions. The free-swimming larva is a Pluteus, very like that of Echinoids (see Fig. 157). Ophiuroids are first found in Silurian strata. The Ophiuroids are usually classified according to the characters of their ossicles and covering plates. Some common genera are Ophiothrix, Ophioconia, Ophiopholis, Ophhira. In the deep-water Astrophyton and Gorgonocephalus the arms are repeatedly branched. In Astronyx loveni, often caught in the trawl off the north coast of Britain, the disc is relatively large and soft and the arms very long. In the extinct Lysophiurae there is an ambulacral groove. Class EcHiNOiDEA. Sea-Urchins, e.g. the common Echinus esculentiis Echinoderms zvith the body covered by rows of plates, usually in vertical series and forming an inflexible test ; the shape of the majority approaches a sphere , but some are pin- cushion-like, flat, or obviously bilateral ; the test is covered with spines which vary greatly in length and thickness in the different types ; the locomotor and respiratory tube-feet usually extend from the peristome to near the aboral pole ; there is often a well- developed system of apical plates ; the mouth is at the lower pole, the anus either at the aboral pole or in the posterior inter-radius ; the gonads are unpaired, five in number, and inter -radial. Description of the Common Sea- Urchin Most sea-urchins live off rocky coasts, and not a few shelter themselves sluggishly in holes. They move by means of their tube-feet and spines, and seem to feed on " acorn-shells " and other small sedentary animals, some seaweeds, and the organic matter found in mud and other deposits. After the perils of youth are past, the larger forms have few formidable enemies. The hard and prickly body is more or less spherical. The food canal begins in the middle of the lower surface ; it ends at the opposite pole in the middle of an apical disc, formed in the young animal of a central plate surrounded by five " ocular " and five " genital " plates. In the adult STRUCTURE OF SEA-URCHIN 3OI the central plate is no longer distinct. Each of the " oculars " has a hole for a sensitive tube-foot ; the genitals bear the apertures of the genital ducts, and one also bears the perforated madreporic plate. From pole to pole run ten meridians of calcareous plates, which fit one another firmly ; five of these (in a line with the ocular plates) are known as ambulacral areas, for through their plates the locomotor tube-feet are extruded ; the five others (in a Ime with the genital plates) are called inter-ambulacral areas, Fig. 163. — Apical disc of sea-urchin. In the centre is the periproct bearing the anus ; around it are five genital or inter-radial plates (g.), one of which is modified as the madreporite (m.) ; between these five ocular or radial plates (o.) ; i r a., an inter-radial or inter-ambulacral area, with spines only ; r.a., a radial or ambulacral area, with .spines and openings for tube-feet. and bear spines, not tube-feet. Altogether, therefore, there are ten meridians, and each meridian area has a double row of plates. On the dry shell from which the spines have been scraped, the ambulacral plates are seen to be perforated by small pores, three pairs or so to each plate. Through each pair of pores a tube-foot is connected with an internal ampulla. In the starfish the ambulacral areas 302 PHYLUM ECHINODERMA are wholly ventral, and the apical area seen on the dorsal surface of the young forms is not demonstrable in the adult. On the shell th6re are obviously many spines, most abundant on the inter-ambulacral areas. Their bases fit over ball-like knobs, and are moved upon these by muscles. But besides these, there are modified spines — {a) several kinds of pedicellarias, with three snapping blades on a Fig. 164. — Dissection of sea-urchin. M. at the lower pole is the mouth ; M. at the upper pole is the madreporic plate ; T.T., one of the large tentacular tube-feet around the mouth ; ■ S.G., a skin-gill ; ST., a standard or perignath ; AL., an alveolus ; R.V., a radial vessel, with ampullae {A.) ; intestine {Int.) fixed by mesenteries tube-feet. P., a pedicellaria ; G., a gonad ; SP., spines ; T.F. mobile stalk, sometimes with apical glands ; and (b) small globular sphaeridia, which show some structural resem- blances to otocysts. It is said that, like true otocysts, they are concerned with the perception of direction of motion. New spines and pedicellariae can be grown to replace those that are shed in unwholesome conditions or rubbed off by accident. This is the only marked regenera- tion in sea-urchins. STRUCTURE OF SEA-URCHIN 303 In front of the mouth project the tips of five teeth, which move against one another, grasping and grinding small particles. They also help locomotion on a flat surface. They are fixed in five large sockets or pyramids, and aiong with five stout " braces " (rotulae) and five curved " compasses " (radii) form " Aristotle's lantern," a masticating apparatus. It surrounds the pharynx, and is swayed about and otherwise moved by muscles, many of which are attached to five beams which project inward from the margin of the shell and form a " girdle " of auriculae, also called standards and perignaths. The shell is covered externally by a dehcate ciliated ectoderm, beneath which, in a thin layer of connective tissue, there is a network of nerve cells. Internally, there is another thin layer of connective tissue, and a ciliated epithelium lining the body cavity. The whole complex test starts from a few triradiate spicules in the Pluteus larva. The skeleton grows by the formation of new plates around the apical disc, and also by the individual increase of each. In a few forms the shell retains some plasticity. The nervous system consists of a ring around the mouth, of radial branches running up each ambulacral area, and of the superficial network. Tube-feet, sphaeridia, pedi- cellariae, and spines are all under nervous control, and each radial nerve ends in the sensitive tube-foot that is pro- truded through each ocular plate. It is probable that all the tube-feet are sensory, and tasting is the main function of ten which lie near the mouth. The alimentary canal passes through Aristotle's lantern, and the intestinal portion lies in two and a half coils around the inside of the shell, to which it is moored by mesenteries. It contains fine gravel, sand, and some organic debris. It ends near the centre of l:he apical disc, whence the pedicellariae have been seen removing the fasces. The spacious body cavity is lined by ciliated epithelium, and contains a " perivisceral " fluid, whose corpuscles have a respiratory pigment (echinochrome). When the fluid of a perfectly fresh sea-urchin is emptied out, the contained corpuscles unite in plasmodia, forming composite amoeboid clots (cf. Protomyxa, etc.). The madreporic plate communicates with a membranous 304 PHYLUM ECHINODERMA Stone canal (calcareous in Cidaris) which runs downwards into a circular vessel near the upper end of the lantern. This gives off five inter-radial transparent " Polian " vesicles and five radial vessels, which run down the sides of the lantern and up each ambulacral area. Each radial vessel gives off numerous lateral branches, which communi- cate with the internal ampullae and thence with the external tube-feet. When the tube-feet are made tense with fluid, they extend far beyond the limit of the spines, and are attached to the surface of the rock over which the sea- urchin slowly drags itself. The sucker at the tip of each tube-foot bears a rosette of small calcareous plates ; indeed, there is hardly any part of an Echinoderm in which lime may not be deposited. Before bending upwards from the base of the lantern, each radial vessel gives off a branch to two large tentacle-like tube-feet without attaching discs. The five pairs lie near the mouth, and are sensitive. The blood vascular system is not readily traced, and there is un- certainty as to many points. A " dorsal or axial organ " lies beside the stone canal, and seems to be connected with a " genital ring " and with a circular vessel around the gullet. There is also a " pseud- hsemal " system consisting of a circum-oesophageal sinus with radial branches. The fluid cannot be distinguished from that of the body cavity ; it contains corpuscles, some of which are pigmented. On the area round about the mouth there are ten hollow outgrowths, which resemble the skin-gills of starfishes. There are also five large vesicles at the top of the lantern (" Stewart's organs ") which may function as internal gills. As already mentioned, the pigmented cells of the body cavity fluid seem able to absorb oxygen. There is no doubt that the water vascular system plays a very important part in respiration. It probably also aids in excretion. The sexes are separate, and indistinguishable externally. Five large branched yellow-brown ovaries or rose-white testes lie inter-radially under the apex of the shell, and open by separate ducts on the five genital plates. In spring the apical disc may be seen covered with orange ova or milky-white spermatozoa. The eggs are fertilised externally by sperms wafted from adjacent sea-urchins, and the free-swimming larva is called a Pluteus. SUB-CLASSES OF ECHINOIDS 305 Classification. -- The class may be divided into three sub -classes or groups of orders. Sub-Class I. Regularia Endobranchiata. Mouth and anus at opposite poles ; the anus surrounded by the apical system of plates if these are developed ; no external gills. e.g. the somewhat primitive Cidaris. Sub-Class II. Regularia Ectobranchiata. Mouth and anus at opposite poles ; a double circle of apical plates surrounds the anus ; there are external gills. e.g. the common genera Echinus, Sirongylocentrotus, Arbacia. The Echinothurinffl have flexible tests and powerful muscles. e.g. Asthenosoma and Phormosoma. Sub-Class III. Irregularia. The anus lies outside the apical system of plates in the posterior inter-radius. e.g. the heart -urchins, Spatangus and Echinocardium, without lanterns. In the related Echinoneus there is a lantern in the young forms. It is interesting to contrast the large massive Clypeaster with the minute Echinocyamus pusiUns, common in the stomach of cod-fishes. Class HoLOTHUROiDEA. Sea-Cucumbcrs Cylindrical or worm-like Echinoderms, elongated in the direction of the main axis^ with more or less tendency to bilateral symmetry, with a usually soft or leathery skin, with irregularly scattered microscopic calcareous bodies, with a terminal month surrounded by tentacles, with a posterior anus, with or without tube-feet, with no external madreporite, with a muscular body wall. The Holothurians do not at first sight suggest the other Echinoderms, for they are Hke plump worms, and the carcareous skeleton is not prominent. But closer examina- tion shows the characteristic pentamerous symmetry, and the occurrence of calcareous plates in the skin. These seem to be absent in the unique pelagic Pelagothuria. Holothurians occur in most seas, from slight to very great depths. Their food consists of small animals, and of organic particles from the sand. Some of them catch these in their waving tentacles, which are then plunged into the pharynx. The muscles of a captured Holothurian often over-contract and eject the viscera at the ends or through a side rupture ; in this way the animal may sometimes escape, and the viscera can be regrown. In Synapta the rupture of the body takes place very rapidly, and is probably defensive, the anterior portion re-forming a complete in- 20 3o6 PHYLUM ECHINODERMA dividual. In some forms of Cucumaria planci the body divides by stricture, torsion, or stretching into two or three equivalent parts, each of which may regenerate the whole. In this case the autotomy seems to be reproductive. The worm-like body is often regular in form, with five equidistant longitudinal bands, along which tube-feet emerge. But three of these " ambulacral areas " may be approximated on a flattened ventral sole, leaving two on the convex dorsal surface, and there are other modifica- Fig. 165. — Spicules of Holothurians.— After Semon. The series 1-6 shows stages in the development of an anchor and a plate in a Synapta. The series A-E shows stages in the development of a wheel in Chiridoia, a Synaptid. Miscellaneous forms, ni-p. tions of form. In many cases the tube-feet are modified into pointed papillae. The body wall is tough and muscular, consisting of epidermis, dermis, and circular muscles, and there are paired longitudinal muscles along each radius. A skeleton is represented by scales, plates, wheels, and anchors of lime scattered in the skin, and by plates around the gullet and on a few other regions. The nervous system consists of a circumoral ring in which the five radial nerves running in the ambulacral areas unite, and from which nerves to the tentacles arise. The ring and the radial nerves are sunk below the skin, Coelomic nervous tissue is developed on the perihaemal STRUCTURE OF SEA-CUCUMBER 307 canals. Sense organs are represented by the tentacles, which sometimes have " ear-sacs " at their bases, and by Fig. 166. — A small sea-cucumber. — From a specimen. T., One of the ten branched tentacles, homologous with tube-feet ; i?., a radial line, corresponding to an internal radial water-vessel, which gives off ampullae internally and tube-feet [T.F.) externally. tactile processes on the dorsal surface of some of the creeping forms. From the terminal or ventral mouth, surrounded by five. 308 PHYLUM ECHINODERMA ten, or more tentacles, the food canal coils to the opposite pole. There it expands in a cloacal chamber sometimes contractile, and from this are given off in many forms a pair of much-branched " respiratory trees," which extend forward in the body cavity. These " trees " are supplied with water by means of the rhythmic contractions of the cloaca. They are respiratory, hydrostatic, and excretory. The body fluid sometimes contains a red pigment like haemoglobin. Arising from the base of the respiratory trees in some Holothurians there are the remarkable " Cuvierian organs," which emit white conical bodies from the cloaca when the animal is irritated. The bodies remain adherent by their bases, are greatly elongated by internal fluid pressure into sticky tubes which break off. They will adhere to almost everything but the Holothurian itself. Those Holothurians, e.g. Holothuria nigra, in which the organs are well developed are often called " cotton- spinners," on account of the dense mass of viscid substance which they eject. A little fish, Fierasfer, introduces itself — tail first — into the cloaca of several Holothurians, and lives there as an innocent commensal. The water vascular system shows many peculiarities. In what, by analogy with the other classes, may be described as the primitive condition, there is a ring canal round the mouth communicating with the exterior by a stone canal, with one or more Polian vesicles hanging in the body cavity, and with five radial canals. The radial canals, as in starfishes and sea-urchins, are connected with internal ampulla? and external tube-feet. The anterior tube-feet are greatly enlarged and modified to form the tentacles which encircle the mouth. It is, how- ever, only rarely that the water vascular system exhibits this primitive condition. In most cases the stone canal loses its original connection with the exterior and opens merely into the body cavity ; often it is represented by numerous small canals, hanging freely in the body cavity (Fig. 167, 5/.). Certain of the tube-feet are always modified to form tentacles, and these may, as in Synapta, be the only representatives of the tube-feet. In regard to the function and degree of development of these, there is indeed much diversity. The blood vascular system consists of a circum-oesophageal ring and vessels to the aUmentary canal and the gonads. The system is in great part lacunar. There is also a pseud-h^mal system. The sexes are usually separate. The reproductive organs do not exhibit radial symmetry, and are branched tubes which open within or just outside the circle of tentacles. Like other internal organs of Holothurians, they are often Pjq^ 157. Dissection of Holothurian {Holothuria tuhulosa) from the ventral surface. t Tentacles surrounding the mouth; tj., scattered tube-feet of '' ventral surface ; c, calcareous ring surrounding the food canal ; a ampullae of tentacles (modified tube-feet) ; r., curcular vessel surrounding the gullet, giving off the branched stone canal (si.), the single Polian vesicle (o.), and the five radial canals (r.c), which run forwards, pass through the calcareous ring, and then curve outwards to run on the surface of the longitudinal muscles a m ) along the radial areas. Of the five longitudinal muscles, one 'only is marked, gi., The gut cut through at the teginmng of the first loop ; m., the mesentery which attaches the gut to the body wall, showing the course of the gut ; g2., the other end of the gut • cl , the cloaca bound down by muscles ; an., the anus • r.t., 'the right respiratory tree— the left is cut short dose to its' origin • ov., the ovary. The blood vessels are not shown. 310 PHYLUM ECHINODERMA very brightly coloured. The larva is, in most cases, what is known as an Auricularia. Sometimes, however, the larval stage is skipped, as in Cucumaria crocea and Psolus ephippifer^ where the eggs and young are attached to the back of the mother. In C. curata the eggs and young are sheltered on the ventral surface ; in C. parva in a shallow ventral insinking ; in C. Icevigata there is an invaginated ventral brood-pouch ; in Chiridota contorta the young are sheltered in the genital tubules ; in Synapta vivipara and some others the body cavity serves as a brood-pouch. This illustrates how the same result may be reached in a great variety of ways. The calcareous plates of Holothurians are found as far back as Carboniferous strata. As " trepang " or " beche-de-mer," the Holothurians of the Pacific form an important article of commerce, being regarded as a delicacy by the Chinese. Classification. — Order i. Actinopoda. The radial water vessels are associated with external tentacles, tube-feet, and ambulacral papilla, but the tube-feet and papillae may be absent. There are several families, e.g. the deep-sea Elasipoda, markedly bilateral, almost always flattened ventrally, often with an external pore for the stone canal, e.g. FApidia and Kolga ; the Aspidochirota), e.g. Holothuria and Stichopus, and Dendrochirota?, e.g. Cucumaria. Thyone, Psolus, with tube-feet as well as tentacles ; the Molpa- diidae with tentacles only, e.g. Molpadia ; the Pelagothuriidae containing the free-swimming Pelagothuria. Order 2. Paractinopoda or Apoda. The only external outgrowths of the water vascular system are the pinnate tentacles around the mouth. One family, Synaptidae, e.g. Synapta and Chiridota. There are no tube-feet or respiratory trees or Cuvierian organs. The calcareous bodies are usually beautiful anchors and plates. ■ Many are hermaphrodite. Class Crinoidea. Feather-stars Usually stalked forms, with five jointed, often branched arms (" brachia "), growing out from a central cup or " theca^^ and bearing pinnules ; the arm.s arise from a corresponding number of thecal plates or " radials,'' below which there is a circlet of alternating " basals,^' often with " infra-hasals " alternating again with them ; below the " basals " or " infra-hasals " there is usually a jointed stem anchored to the substratum by ' cirri. ^^ STRUCTURE OF CRINOID 3" The feather-stars or sea-HUes differ from other Echino- derms in being fixed permanently or temporarily by a jointed stalk. The modern Comatulids, e.g. the rosy feather-star (Comatula or Antedon rosacea), leave their stalk at a certain stage in life ; but the other Crinoids, e.g. Pentacrinus; are permanently stalked, like almost all the extinct stone-lilies or encrinites, once so abundant. Most of them live in deep water, and many in the great abysses. An anchorage is found on rocks and stones, or in the soft Fig. 1 68. — Diagrammatic vertical section through disc and base of one of the arms of Antedon rosacea. — After Milnes Marshall. The section is inter-radial on the left, radial on the right, t., Cili- ated openings in body wall ; h., sub-epithelial ambulacral nerve ; I., water vascular canal ; k., tentacle ; ;-., mouth ; s., intestine ; g., central plexus, with " chambered organ " at its base ; /., coelom; i2l.-/?3., radial plates ; Br., brachial plates ; m., muscle; a., axial nerve-cord ; d., central capsule ; CD., centro-dorsal plate ; p., cirri ; e., nerve branches from central capsule to cirri. mud, and great numbers grow together — a bed of sea- lilies. The free Comatulids swim gracefully by bending and straightening their arms, and ^ they have grappling " cirri " on the aboral side, where the relinquished stalk was attached. By these cirri they moor themselves tem- porarily. Small "'organisms — Diatoms, Protozoa, minute Crustaceans — are wafted down ciliated grooves on the arms to the central mouth, which is of course on the up- turned surface. Some members of the class, e.g. Coma- tula, are infested by minute parasitic " worms " (Myzo- 312 PHYLUM ECHINODERMA stomata) allied to Chastopods, which form galls on the arms. A lost arm can be replaced, and even the visceral mass may be regenerated completely within a few weeks after it has been lost. It has been suggested that the occasional expulsion of the visceral sac frees the Crinoid from para- sites (Dendy). The animal consists of (i) a cup or calyx, (2) an oral disc forming the lid of this cup, (3) the radiating " arms," and (4) the stalk supporting the whole. The lowest part of the cup is supported by a pentagonal " centro-dorsal " ossicle, bearing the cirri ; this conceals the coalesced " basals " of the larva ; above this are three tiers of " radials," whence spring the " brachials " of the arms. The oral disc, turned upwards, is supported by plates. Here the anus also is situated. The arms usually branch in dichotomous fashion, and thus ten, twenty, or more may arise from the original five. But the growing point continues to fork dichotomously, like the leaf of many ferns, and as each alternate fork remains short, a double series of lateral " pinnules " results. The arms are supported by calcareous plates. The stalk usually consists of numerous joints, especially in extinct forms, in some of which it measured over fifty feet in length. Except in Holopus, Hyocrinus, and in the stalked stage of Antedon, the stalk bears lateral cirri. The nervous system consists (a) of a circumoral ring with ambxilac'ral nerves* and (6) of axial coelomic nerves up the ossicles on the opposite side of each arm and connected with a peculiar " chambered organ " in the interior of the centro-dorsal plate. Apart from the superficial epithelium, there are no sensory structures. The ciliated food canal descends from the mouth into the cup, and curves up again to the anus, which is on a papilla. The last part of the gut is expanded to form an anal tube, which during life is in con- stant movement, and has apparently a respiratory function. From the cup, where the body cavity is in great part filled with connective tissue and organs, four coelomic canals extend into each of the arms. They communicate at the apices of the arms and pinnules, and currents pass up one and down the other. The blood-vascular system consists of a circumoral ring, which is connected with a radial vessel under each ambulacral nerve, and with a circum-oesophageal plexus. The water vascular system consists as usual of a circumoral ring and radial vessels, but in several respects it shows remarkable modification. The madreporite of other forms is represented by fine pores which open from the surface of the calyx directly into the body cavity, and which may be very numerous : there are said to be 1500 in Antedon rosacea. By these pores water enters the body cavity, and from it enters the numerous stone canals which hang from the ring freelj' in the body cavity, and open into it near the pore canals. There are no Poliaii vesicles or ampulla; ; the tube-feet are small, arranged in groups of three, and are connected by delicate canals with the radial vessels. Fig. 169.— General appearance of a stalked Crinoid (Pentacrinus). A Pinnate arms; CA., position of calyx; CI., cirri arising in whorls, from the jointed stem {ST.) ; R., attaching root-cirn. SH PHYLUM ECHINODERMA Certain of them form tentacles around the mouth, and these are supphed by canals coming off directly from the ring canal. The sexes are separate. The reproductive organs extend as tubular strands from the disc along the arms, but are rarely functional except in the pinnules, from each of which the elements burst out by one duct in females, by one or two fine canals in males. The oval ciliated larva of Antedon, the only one known, is less peculiar than that of other Echinoderms. There are about 400 living species in twelve genera, but about 1500 species in 200 genera are known from the rocks. The class is re- presented in the Cambrian, and attained its maximum development in Silurian, Devonian, and Carboniferous times. The recent forms include the stalked Pentacrinus, Rhizocrinus, etc., and the free Comatulids, which pass through a stalked Pentacrinus stage, e.g. Antedon. Class Edrioasteroidea. Wholly extinct These extinct Pelmatozoa had a sac-like theca of an indefinite number of irregular plates, with a mouth in the centre of the upper surface, with at most a short stalk. Ordovician, Silurian, and Devonian. " They are alone among Pelmatozoa in presenting a type of ambulacrum from which the holothurian, stellerid, and echinoid types may readily be derived " (F. A. Bather). Class Blastoidea. Wholly extinct The Blastoids are first found in the upper Silurian, later than Cystoids and Crinoids ; they had their golden age in the Carboniferous and Devonian times, but then disappeared. Their body was ovate, with five ambulacral areas, with each groove of which jointed pinnules were associated. Class Cystidea. Wholly extinct The Cystidea are first found in the Lower Silurian rocks, had their golden age in Upper Silurian times, and died out in the Carboniferous period. Their body was ovate or globular, sessile or shortly stalked, covered with polygonal plates often irregularly arranged. Development of Echinoderms The ovum undergoes total segmentation, and a hollow ball of cells or blastosphere results. A typical gastrula is formed by invagination. 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O i3-a •'•.- ii O -13 -S 2 X 43 ryi 3 \: '/I 43 ~ 03 t/l • . 03 -fi ?^ ^' 3 "fl o .- " 'yi Q ^ o CS r^-* '3 y3 CJ w '• 33 oi S aj o 3 J= o a^i g'o CS 03 ■•-' ■—, o .3 in O 43 CJ :3 43 ^ 3 ft 3 OJ CS O ^ ftC/5 -M OJ •- 43 is >, - O .J 1 CS -4-» ■*-> 3 — •5 ^ 5 en CS '^ ~ ^ 3 O 'G oi o .y 3 CS _ b£4; 43 >, OJ ,_, be CS 3 XI Oj OJ -d CJ .3 — tn •J OJ PX ft 4; *" >-. t3 03 -g H 5 3 u> OJ ; ^ 4j ^ I ^ OJ OJ > ^ ., , '-3 4J O c- ft " o x:.t3 ■" X >-?^3 -w ? OJ CS > CS 3i6 PHYLUM ECHINODERMA endoderm into the segmentation cavity ; (b) from the outgrowing of one or more coelom pouches (vaso-peritoneal vesicles) from the gastrula cavity or archenteron. From these vesicles the body cavity and the rudiments of the water vascular system arise. The larva is, first of all, a slightly modified, diffusely ciliated gastrula. In Holothuroids, Echinoids, Asteroids, and Ophiuroids, it becomes quaintly modified b}^ the outgrowth of external processes, and the formation of special ciliated bands. These are at first simply pre-oral and pre-anal rings, but they become drawn out along Fig. 170. — Stages in development of Echinoderms. — After Selenka. I. Section of blastula of Synapta digitata (Holotluiroid), with a hint of gastrulation. 2. Section of gastrula of Toxopneustes brevispinosus (sea- urchin) ; ec, ectoderm ; en., endoderm ; m., segmentation cavity with mesenchyme cells in it. 3. Section of larva of Asterina gibbosa (star- fish) ; BL, blastopore ; g., archenteron ; v.p., vaso-peritoneal vesicle ; r. and /., right and left sides. variously disposed and shaped processes. The larva of Crinoids (of Antedon) is not so divergent. In all cases the bilateral symmetry is preserved. The larva does not grow directly into the adult. On the contrary, the adult arises, for the most part, from new growth within the larva on one side. The arms or pro- cesses peculiar to the larva are absorbed or in part thrown off. Only in a few forms which have brood-chambers or are viviparous is the development direct, and without free- swimming larvae. LarvaB of Echinoderms. — Johannes Midler was the first to show thai the various types of Echinoderm larvae might be derived from one fundamental form. " This fundamental type is an elongated, oval, or pear-shaped larva, DEVELOPMENT OF ECHINODERMS 317 which is somewhat flattened on its ventral side. It has arisen from a gastrula whose blastopore has become the anus, while the archenteron is bent towards the ventral surface, where it communicates by the larval mouth with the exterior. Besides these two apertures, the larva has a third, namely, the dorsal pore of the water vascular system. The ciha, with which the larva was at first uniformly covered, partly disappear, and persist only in restricted regions or cihated bands " (Korschelt and Heider). Crinoids. — The simplest Echinoderm larva is that of Antedon, a somewhat modified oval, with five transverse rings of cilia (the most anterior is less distinct), and a posterior terminal tuft. Eventually the posterior end is elongated to form, in the pentacrinoid stage, an attach- ing stalk, which is afterwards absorbed. As all the extinct Crinoids are permanently stalked, there is here an instance of Recapitulation. Holothuroids.—The larva of Holothuroids (an Auricularia) is much quainter. Its diffuse ciha are succeeded by a wavy longitudinal band, which in the pupa stage breaks into transverse rings, usually five in number. The pre-oral region becomes large. Asteroids. — Nearest the Auricularia is the larva of starfishes, which has the same enlarged pre-oral region. There are two cihated bands, of which the ad-oral is smaller, the ad-anal much larger. They are extended peripherally by the development of soft bilateral arms, and such a larva is known as a Bipinnaria. But another larval form in Asteroids is the Brachiolaria stage, in which three warty arms are formed at the anterior dorsal end, independently of the cihated bands. Ophiuroids and Echinoids.— In the Pluteus larvae (Fig. 157) char- acteristic of these classes the pre-oral region remains small, while the post-anal region becomes large. There is one undulating ciliated band, the course of which is much modified by the growth of six long arms, with temporary calcareous supports. This quaint form is often compared to a six-legged easel. The development of these larval forms into the adult is very intricate. The adult is a new formation within the larva, retaining the water vascular system and mid-gut, but absorbing or rejecting the provisional larval structures. As certain parts are broken down, others are built up, chiefly through the agency of the wandering amoeboid ceUs of the mesenchyme. The first steps in the upbuilding of the adult, and especially of its skeleton, are to some extent parallel in the five classes. One of the most important changes is that from bilateral to radial symmetry. In connection with this, it has been conjectured that the primitive ancestor was bilaterally symmetrical, and that the radiate symmetry was acquired by early sessile or sedentary Echinoderms, such as the Cystoids. As we have already seen, the adults in the different classes tend to acquire an independent and secondary bilateral symmetry. It is very difficult to compare the Echinoderm larvae, even in their simplest form, with those of other animals. The nearest type is perhaps the Tornaria of Balanoglossus, but it again is very peculiar. One naturally tries to compare the Echinoderm larva with the Trocho- sphere of Annelids, but the differences are very marked. One of the most marked of these is the absence of the apical sense organ, so 3i8 PHYLUM ECHINODERMA characteristic of the Trochosphere. The fact that this is represented in the larva of Antedon is regarded by many naturalists as a point of much importance. Relationships of Echinoderma The Echinoderms form an exceedingly well-defined phylum, but the Holothurians especially show how many of the significant char- acters may be lost. In that class we see how the power of forming a calcareous skeleton, the characteristic tube-feet, and the greater part of the peculiar water vascular system, may all disappear ; it is conceivable that further modification of the same kind might eliminate all the dis- tinctively Echinoderm characters, and produce an organism whose systematic position would be very difficult to determine. This is important, because, as we have already seen, there are many " worm- like " types of whose affinities we know nothing. That some of these are related to Echinoderms has been often suggested. It is conceivable that Holothurians of the worm-like Synapta type are nearest the primitive stock of Echinoderma. But there are strong arguments in favour of the view that the free forms, the Eleutherozoa, have been derived from attached Pelmatozoic ancestors. The extinct Edrioasteroidea are in some ways intermediate between the Cystidea and the Eleutherozoa. chaptp:r XIII PHYLUM ARTHROPODA Chief Classes — Crustacea, Prototracheata, Myriopoda, Insecta, Arachnoidea, Pal/eostraca More than half the known species of animals are included in the Arthropod phylum, for of insects alone there are said to be more species than of all other animals taken together. The Arthropods are in some ways like Annelids — in the bilateral symmetry ; in the division of the body into successive segments, some or all of which bear appendages ; in the plan of the nervous system ; and so on. Further- more, Peripatus, which has air-tubes or tracheae somewhat similar to those of Myriopods and Insects, has nephridia like those of some Annelids ; and the biramose appendages of a simple Crustacean like Apus may be compared with the parapodia of an Annelid. It is difficult to discern the relationships of the various classes included in the Arthropod phylum. Crustaceans, most of which are aquatic and breathe by gills, are often opposed to the Prototracheata, Myriopods, Insects, and Arachnoids, most of which are terrestrial or aerial, and breathe by tracheae, or possible modifications of these. Three divergent groups — -the Kiug-crabs (Limulus), and the extinct Eurypterids and Trilobites — may be con- veniently referred to a separate class — Palaeostraca. General Characteristics of Arthropods (to which primitive, parasitic, and degenerate forms present exceptions) The body is bilaterally symmetrical, and consists of numerous segments variously grouped. Several or all of the segments 3'9 320 PHYLUM ARTHROPODA bear paired jointed appendages variously modified. The cuticle is chitinous. Ciliated epithelium is almost always absent. The dorsal brain is connected by a ring round the gullet zvith a double chain of ventral ganglia. Above the food canal lies the heart. The true or primitive coelom is always small in the adult ; the apparent body cavity is of secondary origin^ and has in a great part a blood-carrying or vascular function. The sexes are almost always separate, the reproductive organs and ducts are usually paired. There is often some metamorphosis in the course of development . In habit the Arthropods are predominantly active. Class (Crustacea General Characteristics of Crustaceans (to which primitive, parasitic, and degenerate forms offer exceptions) With few exceptions, e.g. land-crabs, wood-lice, and sand- hoppers, Crustaceans live in water. They breathe by gills or cutaneously. The head carries two pairs of antenna in addition to other appendages, e.g. at least three pairs of jaws ; the thorax, sometimes distinct from, and sometimes fused to the head, bears various kinds of limbs ; the abdomen is usually segmented, and often has appendages. The typical appendage consists of two branches and a basal portion, to which gills may be attached. To the chitin of the cuticle, carbonate of lime is added. A Type of Crustacea. The fresh-water Crayfish {Astacus fluviatilis) (Most of the following description will apply also to the Lobster (Homarus), to the Rock Lobster {Paliniirus), and to the Norway Lobster {Nephrops norvegicus), often called a crayfish.) Mode of life. — The fresh-water crayfish lives in streams, and burrows in the banks. It is not found in Scotland, but occurs here and there in England and Ireland, and is common on the Continent. It is not found in districts where the water contains little lime. The food is very varied — from roots to water-rats ; cannibalism also occurs. STRUCTURE OF CRAYFISH 32 1 The animals swim backwards by powerful tail strokes, or creep forwards on their " walking legs." Their life is tolerably secure, but the frequent moultings during adolescence are expensive and hazardous. When hatched the young are like miniature adults ; for a time they chng beneath the tail of the mother. External appearance. — The head and thorax are covered by a continuous (cephalothoracic) shield ; the abdomen shows obviously distinct segments movable upon one another. As indicated by the appendages, there are three groups of segments or metameres — five in the head, eight in the thorax, six in the abdomen, as well as an un- paired piece or telson on which the food canal ends. Each of the nineteen segments bears a pair of appendages. Among other external characters may be noticed the stalked movable eyes, the two pairs o^ feelers, the mouth with six pairs of appendages crowded round it, and the gills under the side flaps of the thorax. ((i) The external shell or cuticle, composed of various strata of chitin, coloured with pig- ments, hardened with lime salts ; (2) The ectoderm, epidermis, or hypodermis, which makes and remakes the cuticle ; (3) An internal connective tissue layer or dermis, with pigment, blood vessels, and nerves. Internal to this lie the muscles. Between the rings and at the joints the cuticle contains no Hme, and is therefore pliable. It is a layer not in itseL living or cellular, made by the underlying living skin. As it cannot expand, it has to be moulted periodically as long as the animal continues to grow. The old husk becomes thinner, a new one is formed beneath it, a split occurs across the back just behind the shield, the animal with- draws its cephalothorax and then i:ts abdomen, and an empty but complete shell is left behind. The moulting is preceded by an accumulation of glycogen in the tissues, and this is probably utilised in the rapid growth which inter- venes between the casting of the old and the hardening of the new shell. How thorough the ecdysis or cuticle-casting is, may be appreciated from the fact that the covering of the eyes, the hairs of the ears, the lining of the fore-gut and hind-gut, the gastric mill, and the tendinous 21 The Body Wall consists of — 322 PHYLUM ARTHROPODA inward prolongations of the cuticle to which some of the muscles are attached, are all got rid of and renewed. The moults occur in the warm months, eight times in the first year, five times in the second, thrice in the third, after which the male moults twice, the female once a year, till the uncertain limit of growth is reached. It is not clearly known in what form the animals procure the carbonate of lime which is deposited in the chitinous cuticle, but Irvine's experiments have shown that a carbonate of lime shell could be formed by crabs, even when the slight quantity of carbonate of lime in sea-water was replaced by the chloride. Moulting is an expensive and exhausting process, and great mortality is associated with the process itself or with the defenceless state which follows. It is the necessary tax attendant on the advantage of armature. Inequalities in the legs are usually due to losses sustained in combat, but these are gradually repaired by new growth. The surface of the body bears setas or bristles of various kinds. These have their roots in the epidermis, and are made anew at each moult. There are simple glands beneath the gill -flaps, and on the abdomen of the female there are cement glands, the viscid secretion of which serves to attach the eggs. Appendages. — The Hmbs of a Crustacean usually ex- hibit considerable diversity ; in different regions of the body they are adapted for different work ; yet all have the same typical structure, and begin to develop in the same way. In other words, they are serially homologous organs, illustrating division of labour. Typically each consists of a basal piece or protopodtte, and two jointed branches rising from this — -an internal endopodite and an external exopodite ; but in many the outer branch disappears. The protopodite has usually two joints — a basal or proximal coxo- podite, and a distal basipodite ; the five joints which the endopodite frequently exhibits are named from below upwards — ischio-, mero-, carpo-, pro-, dactylo-podites — details of some use in the comparison and identification of species. The stalked eyes are not included in the above list, since their development is not like that of the other appendages ; but cases where an excised eye has been replaced by an antenniform structure suggest that the eye-stalk may be of the nature of an appendage. With many of the thoracic appendages, gills, plate-like epipodites, and setae are associated. It is interesting to connect the structure of the appendages with their functions. Thus it may be seen that the great paddles are fully spread when the crayfish drives itself backwards with a stroke of its tail, while in straightening again the paddles are drawn inwards, and the outer joint of the exopodite bends in such a way that the friction is reduced. THE CRAYFISH 323 THE APPENDAGES OF THE CRAYFISH No. I Name. Function. Structure. Antennules (pre- Tactile, olfactory, Two branches, but possibly oral). with ear - sac at base. not homologous with endo- podite and exopodite. 2 Antennae (pre- oral). Tactile, opening of kidney at base. Small exopodite. 3 Mandibles. Masticatory. Four joints, of which three form the palp (endopodite and upper joint of proto- T3 . podite). OJ'-- 11^ 10 X~^ 4 ist Maxillae. ? Thin single-jointed protopo- dite, small endopodite, no exopodite. 5 2ncl Maxillae. Produces respiratory current. Thin protopodite, filamen- tous endopodite ; the " baler " is formed from the epipodite, probably 6 along with the e.xopodite. ist Maxillipedes ? Thin protopodite, small en- (foot-jaws). dopodite, large exopodite. 7 2nd Maxillipedes. ? Two - jointed protopodite, five - jointed endopodite, long exopodite. 8 3rd Maxillipedes. Masticatory. Two - jointed protopodite, large five-jointed endopo- dite with strong teeth on C3 • its ischiopodite, slender 000 exopodite. H 9 Forceps (chelate). Fighting, seizing. No exopodite. In the claw the last joint bites against 10 Walking Legs (chelate). Walking. a prolongation of the second last. II I) >» Genital opening in female. 12 j» Without chelae. 13 *j Genital opening in )f 14 male. Modified swim- /Serve in the male 1 as canals for the Protopodite and endopodite merets in male ; form a canal ; no exopodite. in female, rudi- \^ seminal fluid. mentary. CI 15 Modified swim- , , All the three parts. merets in male, o~ normal in female T3 — • 16 Swimmerets. / Move slightly like >) < ; 17 )} oars, and carry 1) 18 J 1 1 the eggs in the " ♦ \ female. 19 Great paddles. Important in swim- ming. >> Fig. 171. — Appendages of Norway lobster. Ex., Exopodite ; En., endopodite ; protopodite dark throughout ; Ep., epipodite. I, Antcnnule— £., position of ear; 2, antenna — K., opening of kidney ; 3, mand- ible — P., palp ; 4, first maxilla ; 5, second maxilla — B., baler ; 6, first maxilli- pede ; 7, second maxillipede ; 8, third maxillipede — the basal joint of the proto- podite is called coxopodite, the next basipodite ; the five joints of the endopodite are called— ischiopodite (i.) ; meropodite (w.) ; carpopodite (c.) ; propodite (p.) ; dactylopodite {d.) ; 9, forceps— (7) coxopodite ; (6) basipodite, the jomts of the endopodite are numbered ; 10-13, walking legs ; 14, modified male appendage ; 15-18, small swimmerets ; 19, large paddles. STRUCTURE OF CRAYFISH 325 C.G- O.R SOG :^ ^ V.G It is likely that some of the crowded mouth -parts, e.g. the first maxillEe, are almost functionless. The hard toothed knob which forms the greater part of the mandible is obviously well adapted to its crush- ing work. In connection with the skeleton, the student should also notice the beak (ros/mm) projecting between the eyes ; the triangular area {epistoma) in front of the mouth, and the slight upper and lower lips ; and the lateral flaps of the body wall which project the gills. Each posterior segment consists of a dorsal arch (tergmn), side flaps (pleura), a ventral bar {sternum), while the little piece between the pleuron and the socket of the limb is dignified by the name of epimeron. The hindmost piece {telson), on which the food canal ends ventrally, is regarded by some as a distinct segment. The most difficult fact to understand clearly, is that the cuticle of certain appendages (e.g. the mandibles), and of the ventral region of the thorax, is folded inwards, forming chitinous " tendons " or insertions for muscles, and, above all, constituting the complex, apparently, but not really, internal, " en- dophragmal " skeleton of the thorax, protecting the ventral nerve-cord and venous blood sinus. Muscular system. — The muscles are white bundles of fibres, which on minute examination show clearly that transverse striping which is always well marked in rapidly contracting elements. The muscles are inserted on the inner surface of the cuticle, or on its internal foldings [apodemata). The most important sets are — (i) the dorsal extensors or straighteners of the tail ; (2) the twisted ventral muscles, most of which are flexors or benders of the tail, which have harder work, and are much larger than their opponents; (3) those moving the appendages; (4) the bands which work the gastric mill. Nervous system. — The supra-oesophageal nerve-centres or ganglia, forming the brain, have been shunted far forward by the growth Fig. 172. — Central nervous system of the crayfish. C.G., Cerebral or supra-oesophageal ganglia ; O.R., circum- oesophageal nerve-ring; S.O.G., sub-CEsophageal nerve ganglia, six pairs fused ; V.G., a pair of ventral ganglia ; S.A., the sternal artery passing between the two halves of the ventral nerve-cord ; N., nerves coming from a ventral ganglion. N 326 PHYLUM ARTHROPODA of the pre-oral region. We thus understand how the nerve-ring round the gullet, connecting the brain with the ventral chain of twelve paired ganglia, is so wide. The dorsal or supra-oesophageal ganglia are three-lobed, and give off nerves to eyes, antennules, antennae, and food canal, besides the commissures to the sub-cesophageal centres. They act as a true brain. The sub - oesophageal ganglia, the first and largest of the ventral dozen, inner- vate the six pairs of appen- dages about the mouth. There are other five ganglia in the thorax, and six more in the abdomen. Though the ganglia of each pair are in contact, the ventral chain is double, and at one place, between the fourth and fifth ganglia, an artery (sternal) passes between the two halves of the cord. From each pair of ganglia nerves are given off to appendages and muscles, and apart from the brain these minor centres are able to control the individual movements of the limbs. In the thoracic region the cord is well protected by the cuti- FiG. 173. — Section of compound eye of My sis vulgaris. — After Gren- acher. m., Muscle of eye-stalk ; 1-4, ganglionic swellings in the course of the optic nerve ; n., the nerve librUs passing up to the retinul« ; rh., the rhabdoms ; re., elements of retinulae ; p., band of pigment; c, crystalline cones; co., the corneal facets with the subjacent nuclei. cular archway already referred to From the brain, and from the commissure between it and the sub- oesophageal ganglia, nerves are given off to the food canal, forming a complex visceral or stomato-gastric system. Similarly, from the last ganglia of the ventral chain, nerves go to the hind-gut. If the brain be regarded as the fusion of two pairs of ganglia, as the develop- ment suggests, and the sub-oesophageal as composed of six fused pairs, then these, along with the eleven other pairs of the ventral chain, give a total of nineteen nerve-centres — a pair for each pair of appendages. STRUCTURE OF CRAYFISH 327 Sensory system. — A skin clothed with chitin is not Ukely to be in itself very sensitive, but some of the setae are, and some observers describe a peripheral plexus of nerve-cells beneath the epidermis. The setae are not mere outgrowths of the cuticle, but are con- tinuous with the living epidermis beneath ; and though some are only fringes, both experi- ment and histological examination show that others are tactile. On the under surface of the outer fork of the antennules there are special innervated setae, which have a smelling function. Other specialised setae have sunk into a sac at the base of the anten- nules, and are spoken of as auditory. The sac opens by a bristle- guarded slit on the inner upper corner of the expanded basal joint, and contains a gelatinous fluid and small " otoHths," which appear to be foreign particles. This " ear " seems to be an equili- brating organ, con- cerned with directing the animal's movements. In some other Crustaceans the auditory hairs are lodged in an open depression ; this has become an open sac in the crayfish, a closed bag in the crab. Small setae on the upper lip of the mouth have been said to have a tasting function. Fig. 174. — An ommatidium of a compound eye. CO., cornea; cgn., corneagen layer; n., nuclei; c.c, crystalinfe cones; O.P., outer pigment; 7.P., inner pigment; iJZi., retinule cells ; RH., rhabdom ; N.^ optic nerve. 328 PHYLUM ARTHROPODA The Stalked eyes, which used to be regarded as append- ages, arise in development from what are called " procephalic lobes " on the head. They are compound eyes — that is, they consist of a multitude of elements, each of which is structurally complete in itself. On the outside there is a cuticular cornea, divided into square facets, one for each of the optic elements ; beneath this lie, as in other parts of the body, the nucleated epidermal cells. Then follows a focussing layer, consisting of many crystalline cones. Each crystalline cone is composed of four crystalline cells, which taper internally, and externally secrete a firm crystalline body. The bases of the crystalline cones are surrounded by the retinula cells. Each retinula consists of five elongated cells arranged about a central axis. Distally, this axis is formed by the crystalline cone, proximally by a little rod or rhabdom. The rhabdom consists of four little red rods closely apposed together, and connected by a nerve-fibre with the optic ganglion, which lies at the end of the optic nerve. The proximal ends of the retinal cells are deeply pigmented. Thus each element consists of corneal facet, crystalline cone, and retinula, and the retinula con- sists of internal rhabdom and external retinula cells. Be- tween the individual optic elements lie some pigment cells. The retinular image is erect, not inverted as in the eyes of Vertebrates. Alimentary system. — The food canal consists of three distinct parts — a fore-gut or stomodasum developed by an intucking from the anterior end of the embryo, a hind-gut or proctodaeum similarly invaginated from the posterior end, and a mid-gut or mesenteron, which represents the original cavity of the gastrula. The mouth has been shunted backwards from the anterior end of the body, so that the antennules and antennas lie far in front of it. The fore-gut, which is lined by a chitinous cuticle, includes a short " gullet," on the walls of which there are small glands, hypothetically called " salivary," and a capacious gizzard, which is distinctly divided into two regions. In the anterior (cardiac) region there is a complex mill : in the posterior (pyloric) region there is a sieve of numerous hairs. The mill is very complex ; there are supporting " ossicles " on the walls with STRUCTURE OF CRAYFISH 329 external muscles attached to them, and internally projecting teeth which clash together and grind the food. Three of the teeth are con- spicuous ; a median dorsal tooth is brought into contact with two large laterals. On each side of the anterior part of the gizzard there are two limy discs or gastroliths, which are broken up before moulting, and though quite inadequate to supply sufficient carbonate of lime for the new skeleton, seem to have some relation to this process. The occurrence of chitinous cuticle, setae, teeth, and gastroliths in the gizzard is intelligible when the origin of the fore-gut is remembered, and so is the dismantled state of this region when moulting occurs. AO Fig. 175. — Longitudinal section of lobster, showing some of the organs. H., Heart; A.O., ophthalmic artery; a.a., antennary artery; a.h., hepatic artery; ST., sternal artery; S.A., superior abdominal artery ; M.G., mid-gut ; D.G., digestive gland ; H.G., hmd-gut ; Ex., extensor muscles of the tail ; Fl., flexor muscles of the tail ; I. A., inferior abdominal artery ; G., gizzard ; C, cerebral gangha ; P., pericardium ; T., testes. The mid-gut is very short, but outgrowths from it form the large and complex digestive gland. The mid-gut, here as always, is the digestive and absorptive region, but both processes are carried on to a large extent in the digestive gland, which communicates with the mid-gut by two wide ducts. It is roughly three-lobed at both sides, and consists of an aggregated mass of caeca, closely compacted together. The gland is more than a " liver," more even than a " hepatopancreas." It absorbs peptones and sugar ; like the Vertebrate liver, it makes glycogen ; its digestive juices are comparable to those of the pancreas and the 33© PHYLUM ARTHROPODA Stomach of higher animals. The hind-gut is long and straight. It is lined by a chitinous cuticle, as its origin suggests. There are a few minute glands on its walls. Body cavity. — The space between the gut and the body wall is for the most part filled up by the muscles and the organs, but there are interspaces left which contain a fluid with amoeboid cells. These interspaces seem to represent enlarged blood sinuses (a hasmocoele), rather than a true body cavity or coelom. One of the spaces forms the blood- containing pericardium, or chamber in which the heart lies. Vascular system. — Within this non-muscular peri- cardium, and moored to it by thin muscular strands, lies the six-sided heart, which receives pure blood from the gills (via the pericardium) and drives it to the body. The arterial system is well developed. Anteriorly, the heart gives off a median (ophthalmic) artery to the eyes and antennules, a pair of (antennary) arteries to the antennae, and a pair to the digestive gland (hepatic). Posteriorly there issues a single vessel, which at once divides into a superior abdominal, running along the dorsal surface, and a sternal, which goes vertically through the body. This sternal passes between the connectives joining the fourth and fifth ventral ganglia, and then divides into an anterior and posterior abdominal branch. All these arteries are continued into capillaries. From the tissues the venous blood is gathered up in channels, which are not sufficiently defined to be called veins. It is collected in a ventral venous sinus, and passes into the gills. Thence, purified by exposure on the water- washed surfaces, it returns by six vessels on each side to the pericardium. From this it enters the heart by six large and several smaller apertures, which admit of entrance but not of exit. The blood contains amoeboid cells, and the fluid or plasma includes a copper-containing respiratory pigment, haemocyanin (bluish when oxidised, colourless when de- oxidised), and a lipochrome pigment, called zoonerythrin. Both of these are common in other Crustaceans. The blood has a highly developed power of coagulation, so that slight injuries do not lead to excessive loss. Sensitive " explosive corpuscles " disintegrate readily when the STRUCTURE OF CRAYFISH 33 1 blood is shed, and the plasma surrounding them solidifies in small clots or " islands " ; at a later stage there is a further disintegration of corpuscles and more complete coagulation. Coagulation of the blood is not uncommon in Invertebrates, but is always due to an agglutination, with or without disintegration, of the cells ; only in Crustacea does an actual solidification of the fluid plasma play a part, as in Vertebrates. The blood of Crustacea also resembles that of Verte- brates in that it serves as a vehicle for the transport of specific substances, " hormones " or " internal secretions " from one part of the body to another. Some such hormone has been shown to play a part in the adaptive colour change of the shrimp {Crangon). In Asellus the stimulus for the formation of the brood-pouch seems to be an internal secretion of the ovary. The evidence that true hormones occur in other Invertebrates is not yet decisive. Respiratory system. — Twenty gills — vascular out- growths of the body wall — lie on each side of the thorax, sheltered by the flaps of the shield. A current of water from behind forwards is kept up by the activity of the baling portion, or scaphognathite, of the second maxilla. Venous blood enters the gills from the ventral sinus, and purifled blood leaves them by the six channels leading to the peri- cardium. Observed superficially, the gills look somewhat like feathers with plump barbs, but their structure is much more complex. The most important fact is that they present a large surface to the purifying water, while both the stem and the filaments which spring from it contain an outer canal continuous with the venous sinus, and an inner canal communicating with the channels which lead back to the pericardium and heart. Three sets of gills are distinguishable. To the basal joints of the six appendages, from the second maxillipede to the fourth large limb inclusive, the podobranchs are attached. They come oflf with the appendages when these are pulled carefully away, and each of them bears, in addition to the feathery portion, a simple lamina or epipodite. The membranes between the basal joints of the appendages and the body, from the second maxillipede to the fourth large limb inclusive, bear a second set, the arthrohranchs, which have no epipodites. In Qorinection with the second maxillipede there is a single arthrobranch ; 332 PHYLUM ARTHROPODA in connection with each of the five following appendages there are two ; so that there are eleven arthrobranchs altogether. There remain three pleurobranchs, one on the epimeron of the fifth large hmb, and two others quite rudimentary on the two preceding segments. The bases of the podobranchs bear long setas. In Nepkrops, the podobranchs are represented by a small rudiment on the second maxillipede, and by five well-developed gills on the next five appendages ; there are eleven arthrobranchs, the most anterior being small ; and there are four large pleurobranchs. Excretory system. — A kidney or " green gland " lies behind the base of each antenna, and its open- ing is marked by a con- spicuous knob on the basal joint of that appendage. Each kidney consists of a dorsal sac communicating with the exterior, and of a ventral coiled tube which forms the proper renal organ. The latter is sup- plied with blood from the antennary and abdominal arteries, and forms as waste products uric acid Fig. 176.— Male reproductive organs ^nd greenish guanin. Each of crayfish.-After Huxley. j^-^^^^y ^^^ ^^ regarded ^, Testes; v.d., v^s deferens; i;.^'., open- Vinmnlnoriim with a ing of vas deferens on last walking leg. aS nomOlOgOUS WUn a nephridium. The crayfish has also, near the gills, small branchial glands which excrete carcinuric acid from the blood, and also help in phagocytosis, that important process in which wandering amceboid cells resist infection and help to repair injuries (cf. possible function of thymus in Fishes). In not a few invertebrates there are scattered groups of excretory cells or nephrocytes, and it seems that the endothelial cells of the lymphatic vessels and renal capillaries in tadpoles have a similar function. Reproductive organs. ^ — The male crayfish is distin- guished from the female by his slightly slimmer build, and by the peculiar modification of the first two pairs of abdominal appendages. In both sexes the gonads STRUCTURE OF CRAYFISH 333 are three-lobed, and communicate with the exterior by paired ducts. The testes consist of two anterior lobes lying beneath and in front of the heart, and of a median lobe extending backwards. Each lobe consists of many tubules, within which the spermatozoa develop. From the junction of each of the anterior lobes with the median lobe, a genital duct or vas deferens is given off. This has a long coiled course, is in part glandular, and ends in a short muscular portion opening on the last thoracic limb. The sperma- FiG. 177. — Female reproductive organs of crayfish. — After Suckow. ov.. Ovaries ; ov'., fused posterior part ; od., oviduct ; vu., female aperture on the second walking leg. tozoa are at first disc-like cells ; they give off on all sides long pointed processes like those of a Heliozoon, and re- main very sluggish. The seminal fluid is milky in appear- ance, and becomes thicker in its ^passage through the genital ducts. It is possible that the genital ducts represent modified nephridia, and that the cavities of the gonads are coelomic. The ovaries are like the testes, but more compact. The eggs are liberated into the cavity of the organ, and pass out by short thick oviducts opening on the second pair of walking legs. As they are laid they seem to be coated with the secretion of the cement glands of the abdomen, and the 334 PHYLUM ARTHROPODA mother keeps her tail bent till the eggs are glued to the small swimmerets. Before this, however, sexual union has occurred. The male seizes the female with his great claws, throws her on her back, and deposits the seminal fluid on the ventral surface of the abdomen. The fluid flows down the canal formed by his first abdominal appendages, and these seem Fig. 1 78. — Section through the egg of Astacus after the com- pletion of segmentation. — After Reichenbach. si., Stalk of the egg ; ch., chorion envelope ; bl., peripheral blastoderm within which are the yolk pyramids (dark). to be kept clear by the movements of the next pair, which are also modified. On the abdomen of the female the agglutinated spermatozoa doubtless remain until the eggs are laid, when fertilisation in the strict sense is achieved. The Development has been very fully worked out, and is of interest in being direct, without the metamorphosis so common among the Arthropoda. The spherical ovum is surrounded by a cuticular vitelline membrane, and contains a considerable quantity of yolk. After ferti- DEVELOPMENT OF CRAYFISH 335 Usation the segmentation nucleus divides in the usual way into two, four, eight, and so on, but this nuclear division is not followed by division of the plasma. Eventually the nuclei, each surrounded by a small amount of protoplasm, approach the surface of the egg and arrange themselves regularly round it. The peripheral protoplasm then segments round these nuclei, and thus we have a central core of un- segmented yolk enveloped by a peripheral sphere of rapidly dividing cells. In the central yolk, free nuclei are frequently found ; these are Fig. 179. — Longitudinal section of later embryo of Astacus. — After Reichenbach. Ec, Ectoderm ; m., mesoderm cells ; e.g., cerebral ganglia ; St., stomodaeum ; A., anus ; T., telson ; g., ventral ganglia ; s.s., sternal sinus; ^rf., proctodaeum ; A., heart; w.g.. mid-gut ; yolk pyramids dark. the so-called yolk nuclei. Such a type of segmentation is called peri- pheral or centrolecithal, and is very characteristic of Arthropod eggs. Over a particular region of the segmented egg, known as the " ventral plate," the cells begin to thicken ; at this region an invagination occurs, which represents the gastrula. At the anterior Up of the blastopore the mesoderm appears, being many-celled from the first. Soon the blasto- pore closes ; the cavity of the gastrula thus becomes a closed sac — the future mid-gut. The cells of this archenteron take up the core of yolk into themselves in a way which early suggests their future digestive function. On the surface of the egg there have already appeared ectodermic thickenings — the so-called eye-folds — rudiments of the appendages, and of the thoracic and abdominal regions. 33^ PHYLUM ARTHROPODA In the later stages invaginations of the ectoderm form the fore- and hind-gut, which grow inward from opposite ends to meet the endo- dermic mid-gut. The ear-sac and the greater part of the gills have also an ectodermic origin. From the mid-gut the digestive gland is budded out. The heart, the blood vessels, blood, and muscles are due to the mesoderm. As usual, the nervous system arises from an ectodermic thickening. The eye arises partly from the optic ganglia of the " brain," partly from the " eye-folds," and partly from the epidermis. When the young crayfishes are hatched from the egg-shells, they still cling to these, and thereby to the swimmerets of the mother. In most respects they are like the adults, but the cephalothorax is convex and relatively large, the rostrum is bent down between the eyes, the tips of the claws are incurved and serve for firm attachment, and there are other slight differences. The noteworthy fact is that the development is completed within the egg-case, and that it is continuous without Fig. i8o. — Section through cephalothorax of a crab. — After Pearson, H., Heart ; Te., extension of the terguin ; ST., sternum ; PL., pleuron ; T., tendons ; ist W.L., insertion of first walking leg ; Br., gill in gill- chamber ; g., gut; d.a., descending artery; A., afferent branchial; E., efferent branchial. metamorphosis. The shortened life-history of the crayfish is interesting in relation to its fresh- water habitat, where the risks of being swept away by currents are obviously great ; but it must also be remem- bered that the tendency to abbreviate development is a general one. There is some maternal care in the crayfish, for the young are said sometimes to return to the mother after a short exploration on their own account. The Crab It is instructive to contrast the crab-type with that of the crayfish or lobster. The cephalothorax is broadened by a hollow extension of the gill-covering (branchiostegite) region. The abdomen is greatly reduced, with a soft sternal region, and is bent permanently upwards and forwards in a groove in the thoracic sterna. In the male there are only two pairs of abdominal limbs, which have a reproductive function ; in the female there are four pairs, which carry the eggs. Fig. i8i.— Dorsal aspect of swimming crab {Portimus). P., Paddle; A bd., abdomen; .41., antennules ; .42., autennaj ; £., eyes; F., forceps. Fig. 182. — Dorsal aspect of shore crab {Carcinus). Abd., Abdomen ; A^., antennules ; A^-., antenna; ; E., eyes ; F., forceps. 22 338 PHYLUM ARTHROPODA The eye-stalks lie in sockets of the carapace ; the bases of the refiexed antennules are also in sockets ; the antennae are short and straight. The third maxillipedes are broad and flat and form a kind of oper- culum over the five preceding pairs of appendages. The great claws are relatively very large, the other thoracic legs are non-chelate, and in the swimming crabs, e.g. Portunus (see Fig. i8i), the fifth pair of thoracic legs have their last joint adapted as a paddle. There is a noteworthy change in the nervous system (see Fig. i8i). From the cerebral ganglia a pair of oesophageal commissures extend to a large ganglionated mass sheltered by the endosternal skeleton. It is composed of numerous pairs of ganglia fused together, and gives off nerves to maxilla?, maxillipedes, and thoracic limbs. It is perforated by the sternal artery. The oesophageal commissures are united by a Fig. 183. — Ventral aspect of female shore crab. Abd., Abdomen ; mxp., third maxillipede. transverse commissure just behind the gullet, and in front of this cross junction there are two small ganglia giving off nerves to the mandibles. On the lower surface of the anterior part of the gizzard there are two small gastric ganglia innervated from the cerebrals. When the branchial chamber is opened the large pyramidal gills are seen, also the long sword-shaped epipodite (flabellum) of the first maxillipede which seems to help the " baler," the smaller and mobile epipodites borne by the second and third maxillipedes, and the broad scaphognathite of the second maxilla which bales the water forwards and outwards. It must be clearly understood that the branchial chamber is entirely outside of the body, being formed by the lateral extension of a hollow reduplicature from the tergal region. The large gizzard, the enormous greyish-yellow hepatopancreas, the transparent pericardium, and other organs are readily seen. CLASSIFICATION OF CRUSTACEA 339 Systematic Survey of the Class Crustacea (i) Entomostraca, lower forms. They are usually small and simple. The number of segments and ap- pendages is very diverse. The larva is generally hatched as a simple unsegmented Nauplius. There is no gastric mill. The excretory organ is associated with the second maxillae. (2) Malacostraca, higher forms. They are usually larger and more complex. The head consists of 5, the thorax of 8, the abdomen of 6 (7 in Leptostraca) segments. The larva is usually higher than a Nauplius. There is often a gastric mill. The excretory organ is usually associated with the antennae, but maxillary glands may be present in the larvae, and may even persist in adults. First Sub-Class. Entomostraca Order i. Branchiopoda. — In these at least four pairs of leaf-like swim- ming feet bear respiratory plates segmented, and is protected by a shield-Uke or bivalve shell. The mandibles are without palps, and the maxillce are rudimentary. {a) Phyllopoda. The body has numerous segments and (10- 20 or more) fohaceous ap- pendages with respiratory plates. The shell is rarely absent, usually shield-like or bivalved. The heart is a long dorsal vessel with numerous openings. The eggs can survive prolonged desiccation in the mud. Branchipus, a beautifully colovured fresh - water form, with hardly any shell. A rtemia, Brine - shrimps. Periodically partheno- genetic. Artemia fertilis is one of the four animals known to occur in the dense waters of Salt Lake. A pus, an archaic fresh- water form with a large dorsal shield. The body is generally well Fig. 184. — Dorsal surface of Apus cancriformis. — From Bronn's Thierreich. In the anterior region are the two com- pound eyes, and behind them the simple unpaired eye. The whip-like outgrowths of the first thoracic ap- pendage project laterally. Oral. 340 PHYLUM ARTHROPODA Apus is over an inch in length, a giant among Entomostraca. It has an almost world-wide distribution. The appendages are very numerous and mostly leaf-like. They may be regarded as representing a primitive type of Crustacean limb. Professor Ray Lankester enumerates them as follows : — (1. Antenna. Pre-oral. -! 2. Second antenna. (This is sometimes absent, and y apparently always in certain species.) 3. Mandible. 4. Maxilla. 5. Maxillipede. f6. First thoracic foot (leg-like). Thoracic | 7-16. Other ten thoracic feet (swimmers). (Pregenital). 1 The i6th in the female carries an egg sac or broad- ly chamber. There are eleven thoracic rings on the body. Abdominal f 17-68. Fifty-two abdominal feet, to which there corre- ( Post -genital). \^ spond only seventeen rings on the body. The large dorsal shield is not attached to the segments behind the one bearing the maxillipedes. Many of the thin limbs doubtless function as gills. The genital apertures are on the sixteenth appendages. The anus is on the last segment of the body. There is a pair of ventral ganglia to each pair of limbs ; the ventral nerve-cords are widely apart ; and the cephalic ganglion is remarkably isolated. There is periodic parthenogenesis. (6) Cladocera. Small laterally compressed " water-fleas," with few and somewhat indistinct segments. The shell is usually bivalved, and the head often projects freely from it. The second antennae are large, two-branched, swimming appendages, and there are 4-6 pairs of other thoracic appendages, which, vibrating very rapidly and provided with rows of setae, serve both as a pump and as a sieve on which all the food-particles in the water pumped through are retained. The heart is a little sac with one pair of openings. An excretory organ (the shell or maxillary gland) opens in the region of the second maxillae. It is the Entomostracan equivalent of the antennary green gland of Malacostraca. The males are usually smaller and much rarer than the females. The latter have a brood-chamber between the shell and the back. Within this many broods are hatched throughout the summer. Periodic parthenogenesis (of the "summer ova") is very common. "Winter eggs," which require fertilisation, are set adrift in a part of the shell modified to form a protective cradle or ephippium. Daphnia, Moina, Sida, Polyphemus, Leptodora, and many other " water-fleas," are extraordinarily abundant in fresh water, and form part of the food of many fishes. A few occur in brackish and salt water. In Daphnia the appendages are : — antennules, antennae, mandibles, first maxillae, second maxillae (disappearing in the larva), and five thoracic limbs. The abdomen is turned downwards and forwards, and shows three segments and a telson. " WATER-FLEAS " 341 Order 2. Ostracoda. — Small Crustaceans, usually laterally compressed, with an indistinctly segmented or unsegmented body, rudimentary abdomen, and bivalve shell. There are only seven pairs of appendages : — antennules, antennae, mandibles, first maxillae, second maxillae, and two pairs of thoracic hmbs. Parthenogenesis is often prolonged. Examples. — Cypris (fresh water), Cypndina (marine). Fig. 185. — Daphnia. Eye ; A^., second antenna shell gland ; go., gonad ; brood-pouch ; sp., spine ; men ; t., caudal fork ; g. A^., first antenna ; dg., digestive caeca ; s.g., h., heart in pericardium ; 0., ovum ; B.p., /., furca ; s., setae ; Ab., rudimentary abdo- gut ; /-J, thoracic limbs. Order 3. Copepoda. — Elongated Crustaceans, usually with distinct seg- ments. There is no dorsal shell. There are five pairs of biramose thoracic appendages, but the last may be rudimentary or absent. The abdomen is without limbs, and of its five segments the first two are sometimes united. The females carry the eggs in external ovisacs. Most Copepods move very actively in the water, jerking 342 PHYLUM ARTHROPODA themselves rapidly by means of their thoracic legs, or swim more gently by means of their second antennae. The mandibular palps, Fig. 1 86. — Cypns. M., Marks of adductor muscle ; E., eye seen through the shell {SH.) A.I, first antennae; A. 2, second antennae; F., thoracic legs. Fig. 187. — Cypris, side view, after removal of one valve. — After Zenker. e., Eye ; A.r, first antennae ; A. 2, second antennae ; MN., mandibles ; mx.i, first maxilla ; mx.2, second maxilla ; /./, f.2, thoracic legs ; Ab., rudimentary abdomen. first and second maxilla3, and maxillipedes together form a pump and a fine sieve, on which the food is caught. Many are ecto- parasitic» especially on fishes ("fish-lice"), and are often very COPEPODS 343 degenerate. The free-living Copepods form an important part of the food-supply of fishes. Cyclops, free and exceedingly prolific in fresh water. Its appendages are : — antennules, antennae, mandibles, first maxillae, second maxillae, four pairs of flattened biramous thoracic legs united across the middle with those of the opposite side, another rudimentary pair, and probably the genital valve. Cetochilus, Calanus, free and abundant in the sea. In Chondr acanthus, as in many other cases, the Fig. i88.— Crc/o/)<> type. lA., First antenna; HA., second antenna; OV., ovary; R.S. receptaculum seminis ; OS., ovisac ; F., caudal fork. parasitic females carry the pigmy males attached to their body. Caligus, a very common genus of "fish-lice." In the carp-lic3 {Argulus) the mouth is a sucker with sharp stilets and the second maxillaj form adhesive discs. Lerncea, Penella, etc. The adult females are parasitic, and almost worm-like. The males and th= young are free. Order 4. Cirripedia. — Barnacles and acorn-shells, and some allied degenerate parasites. Marine Crustaceans, w^hich in adult life are fixed head down- wards. The body is indistinctly segmented, and is enveloped 344 PHYLUM ARTHROPODA Fig. 189. — Two barnacles hanging from a ship. In A, some of the biramose thoracic hmbs (cirri) are protruded. C, carina, unpaired ; T; tergum ; SC,, scutum. BARNACLES 345 in a fold of skin, usually with calcareous plates. The anterior antennae are involved in the attachment ; the posterior pair are rudimentary. The oral appendages are small, and in part atrophied. In most there are six (or less frequently four) pairs of two-branched thoracic feet, which sweep food particles into the depressed mouth. The abdomen is rudimentary. There is no heart. The sexes are usually combined, but dimorphic unisexual forms also occur. The hermaphrodite individuals occasionally carry pigmy or " complemental " males. The spermatozoa are mobile, which is unusual among Crustacea. Lepas, the ship-barnacle, is as an adult attached to floating logs and ship-bottoms. The anterior end by which the animal fixes itself is drawn out into a long flexible stalk, containing a cement gland, the ovaries, etc., and involving in its formation the first pair of antennae and the front lobe of the head. The second antennae are lost in larval Hfe. The mouth region bears a pair of small mandibles and two pairs of small maxillae — the last pair united into a lower lip. The thorax has six pairs of two-branched appendages, and from the end of the rudi- mentary abdomen a long penis projects. At the base of this lies the anus. Around the body there is a fold of skin, and from this arise five calcareous plates, an unpaired dorsal carina, two scuta right and left anteriorly, two terga at the free posterior end. The nervous system consists of a brain, an oesophageal ring, and a ventral chain of five or more ganglia. There is a vestige of the nauplius eye. No special circulatory or respiratory organs are known. Two excretory tubes lead from (coelomic) cavities to the base of the second maxilla?, and are probably comparable with shell glands and with nephridia. There is a complete food canal and a large digestive gland. Beside the latter lie the branched testes, whose vasa deferentia unite in an ejaculatory duct in the penis. From the much-branched ovaries in the stalk, the ovi- ducts pass to the first thoracic legs, where they open into a cement - making sac, opening to the exterior. The eggs are found in flat cakes between the external fold of skin and the body. The life-history. Nauplius larvae escape from the egg-cases, and, after moulting several times, become like little Cyprids. The first pair of appendages become suctorial, and, after a period of free- swimming, the young barnacle settles down on some floating object, mooring itself by means of the antennary suckers, and becoming firmly glued by the secretion of the cement glands. During the setthng and the associated metamorphosis, the young barnacle fasts, living on a store of fat previously accumulated. Many important changes occur, the valved shell is developed, and the adult form is gradually assumed. The food consists of small animals, which are swept to the mouth by the waving of the curled legs. Growth is somewhat rapid, but the usual ecdysis is much restricted, except in one genus. Neither the valves, nor the uniting membranes, nor the envelope of the stalk, are moulted, though disintegrated portions may be removed in flakes and renewed by fresh formations. In the allied genu* Scalpellum, some are like Lepas, hermaphrodites, without complementary males {So. 346 PHYLUM ARTHROPODA balanoides) ; others are hermaphrodite, with complementary males (5c. villosum) ; and others are unisexual, but the males are minute and parasitic (5c. regium). Balanus, the acorn-shell, encrusts the rocks in great numbers between high and low water marks. It may be described, in Huxley's graphic words, as a crustacean fixed by its head, and kicking the food into its mouth with its legs. The body is surrounded, as in Lepas, by a fold of skin, which forms a rampart of six or more calcareous plates, and a fourfold lid, consisting of two scuta and two terga. When covered by the tide, the animal protrudes and retracts between the valves of the shell six pairs of curl-like thoracic legs. The structure of the acorn- shell is in the main like that of the barnacle, but there is no stalk. ov Mn Fig. 190. — Acorn-shell {Balanus tintinnabulum). — After Darwin. r., Tergum ; CR., thoracic legs ; R., outer shell in section ; D., aper- ture of oviduct ; F., mantle cavity ; X., depressor muscle of tergum ; AN., antennae ; OV., ovary ; G., depressor of scutum ; /f., oviduct ; ^.M., adductor muscle of scuta ; S., scutum. The life-histor}' also is similar. A Nauplius is hatched. It has the usual three pairs of legs, an unpaired eye, and a delicate dorsal shield. It moults several times, grows larger, and acquires a firmer shield, a longer spined tail, and stronger legs. Then it passes into a Cypris stage, with two side eyes, six pairs of swimming legs, a bivalve shell, and other organs. As it exerts itself much but does not feed, it is not unnatural that it should sink down as if in fatigue. It fixes itself by its head and antennae, and is glued by the secretion of the cement gland. Some of the structures, e.g. the bivalve shell, are lost ; new structures appear, e.g. the characteristic Cirriped legs and the shell. Throughout this period, which Darwin called the " pupa stage," there is external quiescence, and the young creature continues to fast. The skin SACCULINA 347 of the pupa moults oft" ; the adult structures and habits are gradually assumed. At frequent periods of continued growth the lining of the shell and the cuticle of the legs are shed. In spring these glassy cast coats are exceedingly com- mon in the sea. Acorn-shells feed on small marine animals. They fix themselves not to rocks only, but also to shells, floating objects, and even to whales and other animals. On the ventral surface of the abdomen of crabs, Saccidina, one of the most degenerate of all parasites, is often found. Its history has been beauti- fully worked out by Professor Delage. It is in shape an ovoid sac, and is attached about the middle of a segment. On the lower surface of the sac there is a cloacal aperture, opening into a large brood-chamber, usually distended with eggs contained in chitinous tubes. The brood- chamber surrounds the central " visceral mass," consisting of a nerve ganglion, a cement gland which secretes the egg-cases, and the hermaphrodite reproductive organs ; of digestive or vascular systems there is no trace. The parasite is attached by a pe- duncle, dividing up into numer- ous " roots," which ramify within the body of the crab, and by them the Sacculina ob- tains nutrition and gets rid of its waste products ; it is practically an endoparasite. The larvae leave the brood-chamber as Nauplii ; they moult rapidly and become Cyprid larvae. These fix themselves by their antennae to young crabs, at the uncalcified membrane round the base of large bristles. The thorax and abdomen are cast off ; the structures within the head region contract ; eyes, tendons, pigment, the remaining Fig. 191. — Development of Sac- culina. — After Delage. (Not drawn to scale.) A, Free-swimming Nauplius, with three pairs of appendages ; B, pupa stage ; C, adult protruding from the abdomen of a crab. 348 PHYLUM ARTHROPODA yolk and the carapace, are lost ; a little sac remains, which passes into the interior of the crab. It reaches the abdomen, and, as it approaches maturity, the integuments of the crab are dissolved beneath it, and Fig. 192. — Sacculina as a parasite on a female crab. — After Delage. The abdomen of the crab has been drawn back, showing the bean-shaped Sacculina. A., Anus of crab ; R., reproductive aperture of the parasite. The dark-coloured branches are the absorbent root-like processes of Sacculina, supposed to be visible through the crab's shell. They extend even into the limbs. They are shown on one side only. the sac-like body protrudes. It appears to live for three years, during which time the growth of its host is arrested, and no moult occurs. In some cases the parasitised male crab puts on feminine characters, and the testis is replaced by egg-producing tissue. In a parasitised CLASSIFICATION OF CRUSTACEA 349 Fig. 193. — Nebalia. — After Sars. 1 .-X ,, first antennae; A. 2, second antennae ; limbs ; Ab.4, Ab.d, fourth and sixth abdominal hmbs. SH., Shell; A.;, first antenna 7 .;4-^ r.c.^'^d antenn^j^ TH., 8 tnoracic Fig. ig4.—Anaspides.—Aiter Caiman. A / A ^. Antennse ; Ex., rudimentary exopodite ; G., respiratory lamina ' ' PR? PES seventh and eighth thoracic limbs or pereiopods ; PL.f, 2 6 first second, and sixth abdominal limbs or pleopods. 350 PHYLUM ARTHROPODA female the ovaries may become quite degenerate. Forms allied to Sacculina are grouped together as Rhizocephala. One of them — • Sesarmaxenos — occurs on a fresh - water crab, Sesarma, in the Andamans ; all the rest are marine. Second Sub-Class. Malacostraca Series I. Leptostraca. Division Phyllocarida. Marine Crustaceans of great systematic interest, retaining in many ways the simplicity of ancestral forms, and linking Malacostraca and Entomostraca. The most important genus is Nebalia (Fig. 193). A bivalve shell covers the whole of the lank body, except the last four abdominal segments ; the head is free from the thorax ; the eight segments of the thorax are free from one another, and the plate-like appendages resemble those of Branchiopods ; the abdomen has seven segments and a telson with two forks ; the elongated heart extends into the abdomen, and has seven pairs of lateral apertures or ostia. There are both antennary and maxillary excretory organs. Nebalia and its congeners are probably related to certain ancient fossil forms from Palaeozoic strata, e.g. Hymenocaris from the Cambrian. Series II. Eumalacostraca. Division i. Syncarida, the order Ana- spidacea, primitive fresh-water forms, without a carapace ; with the eight thoracic segments all distinct (Anaspides), or with the first one fixed to the head [Koonunga) ; with stalked eyes in Ana- spides, sessile eyes in Koonunga ; with lamellar branchiae on the thoracic legs, whose slender exopodites are also res- piratory (Fig. 194). Division 2. Peracarida, with a carapace that leaves at least four of the thoracic segments free, with the first thoracic seg- ment always fused to the head, with usually sessile eyes, with a brood-pouch on the thoracic appendages of the female, with an elongated heart, with direct de- velopment. Numerous orders including : — the pelagic Mysidacea (formerly united with Euphausiacea as Schizopods), e.g. Mysis ; the pelagic and deep-water Cumacea, e.g. Cuma and Diastylis ; the Isopods, with dorso-ventral flattening of the body, a posterior heart, and respira- tory organs on the abdominal limbs, e.g. the terrestrial wood-lice {Porcellio, Onis- cus, etc.), which show minute trachea-like Fig. 195. — An Amphipod (Caprella linearis). The two anterior thoracic seg- ments are fused to the head ; the abdomen is greatly re- duced and without append- ages ; the fourth and fifth thoracic segments bear only respiratory plates. CLASSIFICATION OF CRUSTACEA 351 respiratory tubes in the abdominal limbs, and corresponding forms on the shore {e.g. Ligia, Idotea) ; the Amphipods, with lateral flattening ABD Fig. 196. — Dorsal view of a wood-louse {Oniscus), a typical Isopod, but of terrestrial habits— after Webb and Sillem. The abdominal appendages or pleopods are penetrated by minute air-containing tubes which form a respiratory system adapted for terrestrial con- ditions. In the marine Isopods, the endopodites of the abdominal appendages are gill-like and serve for aquatic respiration. The number of segments and appendages is like that in the crayfish. H., Head. ABD., A segment of the abdomen. A., Antenna. TH., A segment of the thorax. A. P., Last pair of abdominal appendages. of the body, an anterior heart, and respiratory organs usually on the thoracic limbs, e.g. Gammarus locusta in the shore pools, G. pulex in 352 PHYLUM ARTHROPODA fresh water, and sandhoppers like Talitrus and Orchestia : the " no body " crabs, Caprella ; Phronima, living inside the glassy case of the free-swimming Tunicate Pyrosoma. Division 3. Hoplocarida, with a carapace that leaves at least four of the thoracic segments free, with stalked eyes, with the eggs carried in a chamber formed by the maxillipedes, with an elongated heart. abd Fig. 197. — Hermit-crab withdrawn from its shell. The anterior appendages are broken off. hd., Head ; th., thorax ; abd., abdomen. and with a complicated metamorphosis. Order : — Stomatopods, e.g. SqiiiUa, with the second maxillipedes forming very large raptorial organs. Division 4. Eucarida, with a cephalo-thoracic shield uniting the head and thorax segments ; with stalked eyes ; with a saccular heart ; with eggs attached to the abdominal endopodites ; with spherical spermatozoa showing peculiar radiating pseudopodia ; usually with a complex metamorphosis. CLASSIFICATION OF CRUSTACEA 353 Order i. Euphausiacea : — shrimp-like surface and deep-water forms, with biramous thoracic limbs as in Mysids, e.g. Euphausia. Order 2. Decapoda : — with the three anterior thoracic limbs turned forward as maxillipedes, with the other thoracic limbs almost always uniramous. Sub-order Macrura. — Abdomen long. Homarus (lobster) ; Neph- rops (Norway lobster, sea crayfish) ; Astacus (fresh-water crayfish) ; Palinurus (rock lobster), whose larva was long known as the glass- crab (Phyllosoma) ; Penceus, a shrimp which passes through Nauplius, Zosa, and Mysis stages ; Lucifer and Sergestes are also hatched at a stage antecedent to the Zoaea ; Crangon vulgaris (the British shrimp) ; Palcemon, Pandalus, Hippolyte (prawns) ; Galathea (with the abdomen bent forwards) ; Pagurus, Etipagurus (hermit-crabs) ; Birgus latro (the terrestrial robber or palm-crab), in which the upper part of the gill- FiG. 198. — Mysis flexuosa, from side. 6., Brood-pouch borne on posterior thoracic limbs ; 0., otocyst in tail. Note eight pairs of similar biramose thoracic feet. The last two thoracic segments are not covered by the shield. cavity is shut off to form a "lung," the walls having numerous vascular plaits. Sub-order Brachyura. — Abdomen short, and bent under the thorax. It is narrow in the male, and does not usually bear more than two pairs of appendages ; it is broader in the female, and bears four paired appendages. The ventral ganglia have fused into an oval mass. Cancer (edible crab)j Carcinus mcenas (shore- crab) ; Portunus (swimming crab) ; Dromia (often covered by a sponge) ; Pinnotheres (living inside bivalves) ; Telphusa (a fresh- water crab) ; Gecarcinus (land-crabs, only visiting the sea at the breeding season). History. — Fossil Crustaceans are found in Cambrian strata, but the highest forms (Decapoda) were not firmly established till the Tertiary period. Some of the genera, e.g. the Branchiopod Estheria, living from Devonian ages till now, are remarkably persistent and successful. How the class arose we do not know ; it is probable that types like Anaspides and Nehalia give us trustworthy hints as to the ancestors of the higher 23 354 PHYLUM ARTHROPODA Crustaceans ; it is likely that the Phyllopods, e.g. Apus, bear a similar relation to the whole series ; the Copepods also retain some primitive characteristics ; but it is difficult to say anything definite as to the more remote ancestry. We naturally think of a segmented worm-type as a plausible starting- point for Crustaceans, and it is not difficult to imagine how a develop- ment of cuticular chitin would tend to produce a flexibly jointed limb out of an unjointed parapodium ; how the mouth might be shunted a little backwards, and two appendages and ganglia a little forwards ; and how division of labour would result in the differentiation of distinct regions. General Notes on Crustaceans Of a class that includes animals so diverse as crabs, lobsters, shrimps, " beach-fleas," *' wood-lice," barnacles, acorn-shells, and " water-fleas," it is difficult to state general characteristics, other than those facts of structure which we have already summarised. Admitting the parasitism of many Crustaceans, and the sedentary life of barnacles and acorn-shells, we must still allow that great activity characterises the class. With this may be connected the brilliant colouring, the power of colour change, and the phosphorescence of many forms. Except in the case of a few primitive and degenerate forms, the Crustacea are all segmented. In this, in the presence of hollow jointed appendages, in the reduction of the coelom, and in their firm chitinous cuticle, the Crustacea resemble other Arthropods ; as special char- acteristics we notice the two pairs of antennae, the presence of carbonate of lime in the cuticle, and the nature of the respiratory organs — these, with few exceptions, being adapted for breathing in water. While these characters remain constant throughout the group, there is an almost infinite variety in detail. In regard to the segmentation of the body, we notice that, apart from the general tendency to reduction which is so marked in many parasitic forms, the higher forms as compared with the lower show marked specialisation. In the primitive Phyllopods the body con- sists of a large but varying number of segments, remarkably uniform in structure. The higher Crustacea, on the other hand, are characterised by their relatively few but constant segments, which exhibit marked division of labour ; a CRUSTACEAN CHARACTERS 355 comparison of Nebalia, Mysis, Euphausia, Penaus, Nephrops, will make this plain. The same gradual process of specialisation is observable in the appendages. Typically consisting of a basal piece and two branches, the append- ages, like the parapodia of Annelids, are primitively organs of locomotion, usually adapted as swimming organs. In Phyllopods the great majority of the append- ages remain perman- ently at this level. It is worth notice that in the Nauplius and in Ostracods and in free- swimming Copepods, the antennae themselves are swimming organs. Just as, however, in the Annelid head the loco- motor function of the parapodia becomes subordinated to the sensory one, so also in Crustacea the anterior appendages of the head become specialised as Fig. 199. — Nervous system of shore-crab sense organs. Again, {Cardnus m^na5).— After Bethe. the appendages in con- *''•. The supra-cesophageal mass; g., gullet nection with the mouth become modified in connection with ali- mentation, and the further processes of specialisation which differentiate the regions of the body are reflected in the appendages of these regions. It is this specialisation of certain appendages to function as mastica- tory organs which especially characterises Arthropods as compared with Annelids. In the nervous system there is always a certain amount of fusion of ganglia — these never being so nunierous as the segments — but the fusion is more marked in the more specialised forms. In the Crabs the ventral chain is repre- surrounded by g.r., the gullet ring ; m., the sub-cesophageal mass representing a fusion of the thoracic ganglia of the crayfish, and giving off nerves to the limbs ; behind it is a short strand representing the abdominal ganglia of the crayfish. a^., Antennules ; a'"., antennae ; e., eye. oc6 PHYLUM ARTHROPODA sented by a lobed ganglionic mass in the thorax, connected with a mere rudiment, which corresponds to the abdominal portion of the cord in the crayfish (Fig. 199). Sense organs are usually well developed, and are not confined to the head region ; thus many Mysids have " auditory " organs in the tail (Fig. 198). The alimentary canal runs straight throughout the body ; it consists of fore-gut, mid-gut, and hind-gut. The fore-gut and hind-gut are anterior and posterior invaginations of ectoderm, and are always large, especially in Malacostraca. In the higher Malacostraca the fore-gut is furnished with a gastric mill. The mid-gut or archenteron is always short, but has con- nected with it diverticula which form the so-called hepato- pancreas. In the Entomostraca there is usually only a single pair of outgrowths ; in Mysids, Cumacea, and larval Decapods there are three pairs ; a process of rapid growth and branching converts these into the compact digestive gland of the adult Decapods. In connection with the posterior end of the mid-gut in Amphipods and some others, there is a pair of blind tubes functioning as excre- tory organs, and presenting an interesting similarity to the Malpighian tubes of insects, which, however, are in con- nection with the hind-gut. The body cavity is never large, being mainly filled up with muscles and organs, and, as in Arthropods in general, the true coelom is virtually absent. In the blood, haemocyanin is the commonest pignient, but is not universal. Respiration is carried on in many different ways. In the simple forms it may be merely by the general surface, but in the majority of cases, certain portions of the limbs, or outgrowths of the limbs, constitute definite respiratory organs, often specialised to form gills. In the excretory system the numerous nephridia of Annelids are absent. The typical excretory organs of the Entomos- traca are the " shell glands " — paired coiled tubes opening on the second maxilla ; of the Malacostraca, the antennary glands exemplified by the green glands of the crayfish. The genital ducts are possibly modified nephridia. There are many peculiarities connected with reproduc- tion — 'thus parthenogenesis for prolonged periods is common among '* water-fleas " ; hermaphroditism is frequent, occurring, for example, in barnacles, acorn- DEVELOPMENT OF CRUSTACEA 357 shells, etc., and it is often complicated by the simultaneous existence of " pigmy " complemental males. When separate, the two sexes are often very diverse. The sper- matozoa are often exceptional in being very slightly motile. Some appendages are often modified for copula- tion or for carrying the eggs. Development. — The ova of most Crustacea show con- siderable similarity to those of Astacus, and the segmenta- tion is typically of the kind already described. But while Fig. 200. — Partial peripheral segmentation of the ovum — characteristic of Arthropods. A. A vertical section of the segmented ovum, showing some rather larger segmenta- tion cells, the macromeres [MA.], and some rather smaller cells, the micromeres (MI.). The nuclei of the cells are indicated by dark spots. BL., Blastomeres, the term applied to the segmentation-cells in general, whether macromeres or micromeres, or all equal. B. A surface view of the same ovum, showing macromeres (MA.) and micromeres (MI.). The surface view of an ovum showing this type of segmentation is like the fruit of rasp or bramble. this is the most typical case for Crustacean, and, indeed, for Arthropod development, it is possible, within the Hmits of the class Crustacea, to trace out a complete series, in which the first term is a segmentation of the complete and equal type, like that of a worm, and the last the purely peripheral. In the same way, though gastrulation is usually much disguised, there are many modes, from an invagination of the simplest embolic type (Lucifer), and through the condition described for Astacus, to the forma- tion of endoderm by the ingrowth of a solid plug of cells. 3S8 PHYLUM ARTHROPODA Compared with Astacus, however, the most important point we have to notice is the frequent occurrence of a very striking metamorphosis in the hfe-history. In other words, the larva hatched from the egg is rarely like the parent, and only acquires the adult characters after a series of pro- found changes. Tn some types {Nebalia, My sis) a meta- morphosis takes place within the egg-case, and in the few forms in which development seems to be direct, slight traces of metamorphosis are found. Almost all the lower Crustaceans and some higher forms, e.g. Eu- phaiisia and Penceus, are hatched in a Nauphus stage. In the remaining types the Nauplius stage is indicated within the egg by the moulting of a larval cuticle (as in Astacus). The Nauplius is characterised by a typically rounded body, and by the presence of three pairs of appendages, which are the only ob- vious indications of seg- mentation. The first pair of appendages are un- branched, and bear larval sense organs, the next two are biramose swim- ming organs. There is an unpaired median eye, but no heart, and frequently no hind-gut. The three pairs of ap- pendages become the first and second pairs of antennae and the mandibles of the adult. The head region of the Nauplius becomes the head region of the adult ; the posterior region also persists ; the new growth of segments and appendages takes place (with numerous moultings) in the region between these. Fig. 20I. — Zoasa of common shore- crab {Carcinus mcenas). — After Faxon. The appendages are numbered ; c. i., alimentary canal. gills ; CRUSTACEAN LARVi^ 359 The second important form of larva is the Zoaea, which has all the appendages on to the last maxillipedes inclusive, a segmented abdomen, and two lateral compound eyes, in addition to the unpaired one of the Nauplius stage. Most Decapoda are hatched in the Zoaea stage. (a) The crayfish {A stacus) is hatched almost as a miniature adult. The development is therefore very direct in this case. (6) The lobster {Homarus) is hatched in a Mysis stage, in which the thoracic limbs are two-branched and used for swimming. After some moults it acquires adult characters. {e) Crabs are hatched in the Zocbu form, and pass with moults through a Megalopa stage, with the abdomen in a line with the cephalo- thorax. The abdomen is subsequently tucked in under the thorax. {d) PencBus (a kind of shrimp) is hatched as a Nauplius, becomes a Zocea, then a Mysis, then an adult. Its relative Lucifer starts as a Meta-Nauplius with rudiments of three more appendages than the Nauplius. Another related form, Sergestes, is hatched as a Protozocea, with a cephalothoracic shield and an unseg- mented abdomen. Thus there are two grades between Nauplius and Zoa?a. Three facts must be borne in mind in thinking over the life -histories of crayfish, lobster, crab, and Penceus : (i) There is a frequent tendency to abbreviate development, and this is of more importance when meta- morphosis is expensive and full of risks ; (2) there is no doubt that larvas exhibit characters which are related to their own life rather than to that of the adult ; {3) it is a general truth, that in its individual development the organism recapitulates to some extent the evolution of the race, that ontogeny tends to recapitulate phylogeny. But while there can be no doubt that the metamorphosis of these Crustaceans is to some extent interpretable as a recapitulation of the racial history — for there were unsegmented animals before segmented forms arose, and the Zocza stage is antecedent to the Mysis, etc.— yet it does not follow that ancestral Crustaceans were like Nauplii. On the contrary, the Nauplius must be regarded as a larval reversion to a type much simpler than the ancestral Crustacean. Ecology. — Most Crustaceans are carnivorous and pred- atory ; others feed on dead creatures and organic debris in the water ; a minority depend upon plants. Many of the smaller forms play a very important part in the economy of nature — in the circulation of matter — for while they feed on animalcules and debris, they are themselves the food of larger animals such as fishes. Parasitism occurs in over 700 species, in various degrees, and, of course, with varied results. Most of the parasites keep to the outside of the host {e.g. fish-lice), and suck nourishment by their mouths ; the Rhizocephala {e.g. 360 PHYLUM ARTHROPODA Sacculina) send ramifying absorptive roots through the body of the host. Sometimes the parasitism is temporary (Argulus) ; sometimes only the females are parasitic (e.g. in Lerncea). The parasites tend to lose appendages, segmen- tation, sense organs, etc., but the reproductive organs become more fertile. The hosts, e.g. crabs, infested by Rhizocephala, are sometimes materially affected, and even rendered incapable of reproducing. Some Crustaceans live not as parasites, but as commensals with other animals, doing them no harm, though sharing their food. Thus there is a constant partnership between some hermit-crabs and sea-anemones (Fig. 113). The hermit-crab is concealed and protected by the sea-anemone ; the latter is carried about by the Crustacean, and gets fragments of food. Masking is also common, especially among crabs. Some will cut the tunic off a sea-squirt and throw it over their own shoulders. Many attain a mask more passively, for they are covered with hydroids and sponges, which settle on the shell. There is no doubt, however, that some actively mask themselves, for besides those known to use the Tunicate cloak, others have been seen planting seaweeds on their backs. The protective advantage of masking both in offence and defence is very obvious. The intelligence of crabs and some of the higher Crus- taceans is well developed. Maternal care is frequent. Fighting is very common. Many will " voluntarily " part with a leg to save themselves from their enemies. The loss of limbs is readily repaired. Deep-sea Crustaceans are very abundant, and often remarkable " for their colossal size, their bizarre forms, and brilliant red colouring " ; in many cases, they are brilliantly phosphorescent. Yet more abundant are the pelagic Crustaceans (especially Entomostraca and Mysids) ; they are often transparent except the eyes, often brightly coloured or phosphorescent. Many Crustaceans live on the shore, and play a notable part in the struggle for existence which is so keen in that densely crowded region. The lower Crustaceans are abundantly represented in fresh water, in pools, streams, and lakes. A few Crus- taceans, such as wood-lice and land-crabs, are terrestrial, and some blind forms occur in caves, CHAPTER XIV PHYLUM ARTHROFOD A— {continued) Classes {continued)— Onyckofhora or Prototracheata ; Myriopoda ; and Insecta These three classes form a series of which winged insects are the cUmax. The type Peripatus is archaic, and hnks the series to the AnneUds : the Myriopods lead on to the primitive wingless insects. All breathe by tracheae — tubes which carry air to the organs of the body — and all have antennae ; hence they are often united under the title Tracheata Antennata. First Class of Tracheata Antennata.— On ychophora or Prototracheata General Characters The body is worm-like in form, soft-skinned, and without external segmentation. The appendages are — a pair of prominent pre-oral antennae, a pair of jaws in the mouth, a pair of slime-secreting oral papillce, which development shows to be true appendages, numerous pairs of short, imperfectly jointed legs, each with two claws, and a pair of anal pqpillce, which are rudi- mentary appendages. The legs contain peculiar {crural) glands. Respiration is effected by numerous unbranched tracheee with openings irregularly scattered. The heart is an elongated dorsal vessel with valvular ostia. There is a series of nephridia in the legs. The halves of the ventral nerve-cord are widely separate. All are viviparous. In its possession of tracheee and nephridia this type is an interesting connecting link ; in many ways it seems to be an 361 362 PHYLUM ARTHROPODA old-fashioned survivor of an archaic stock. There are about half a dozen genera very luidely distributed. The Onychophora'are very beautiful animals. Prof. Sedgwick says : " The exquisite sensitiveness and continu- ally changing form of the antennae, the well-rounded plump body, the eyes set like small diamonds on the side of the head, the delicate feet, and, above all, the rich colouring and velvety texture of the skin, all combine to give these animals an aspect of quite exceptional beauty." They are shy and nocturnal, with a great dislike to light. They seek out damp places under leaves and among rotting wood. They feed on insects, which they catch by the ejection of slime from the oral papillae. The slime is also squirted out when they are irritated. To their shy habits their persistence is possibly in part due. They are able to move quickly, some- what after the fashion of millipedes, especially like Scolopendrella. They have been seen to climb up vertical glass plates. When at rest or irritated they coil up in a circle. Form. — The body suggests an Annelid or a caterpillar, but, apart from the appendages, there is no external segmentation. There is a clear dorso-median line. Over the soft skin are numerous minute warts with small bristles. The mouth is ventral and anterior ; the anus terminal and posterior. Appendages.— The first are the large, ringed antenna3 ; then follow the sickle-like jaws in the mouth cavity ; a little farther back are two oral papillae from which slime is exuded. Then there are the 14-42 stump-like legs, each with two terminal chitinous claws. Skin. — The chitinous cuticle, ordinarily thick in Arthropods, is delicate. It is subject to moulting. The epidermis is a single layer of cells. Beneath it there is a dermis. Muscular system. — Externally there is a layer of circular muscles ; within this lies a double layer of diagonal fibres ; internally there are strong longitudinal bundles. Finally, in connection with this internal layer, there are fibres which divide the apparent body cavity into a median and two lateral compartments. The median includes heart, gut, slime glands, reproductive organs ; the laterals include the nerve- cords and salivary glands ; the legs contain nephridia and coxal or Fig. 202. — External form of Peripatus. — After Balfour. Note antennae and simple legs. STRUCTURE OF PERIPATUS 363 crural vesicles. Striped, rapidly contracting muscles are characteristic of Arthropods, but in Peripatus the muscles are unstriped, excepting those which work the jaws and are perhaps the most active. The true ccelom is represented in the embryo by the cavities of the mesoderm segments, which give origin to the muscular system. Nervous system. — The dorsal brain is connected by an oeso- phageal ring with the two widely separate latero-ventral nerve-cords. These are connected transversely by numerous commissures, are shghtly swollen opposite each pair of legs, to which they give off nerves, and are united posteriorly over the anus. There are only hints of ganglia, but there is a continuous layer of ganglionic cells. The brain is very homogeneous, simpler than that of most Insects. Sense organs are sl.g. oe.co Fig. 203. — Dissection of Peripatus. — After Balfour. at., Antennas ; or. p., oral papillae ; e.g., cerebral ganglia ; sl.d., duct of slime gland {sl.g.) ; s.o.S, eighth segmental organ or nephridium ; V.C., ventral nerve connected by transverse commissures (co.) with its fellow ; s.0.17, seventeenth nephridium ; g.o., genital aperture ; A., anus; p.d.c, posterior commissure; F./j, seventeenth appendage ; a.g., last crural gland — that of the opposite side is marked v.g. ; F./, F.2, first and second legs ; oe.co., oesophageal nerve commissure ; oe., oesophagus ; ph., pharynx — the remainder of the gut is removed. represented by two simple eyes on the top of the head. These are most like the eyes of some marine Annelids. Alimentary canal. — Round about the mouth papillae seem to have fused to form a " mouth cavity," which includes the mandibles, a median pad or tongue, and the opening of the mouth proper. The mouth leads into a muscular pharynx, into which opens the common duct of two large saUvary glands, which extend far back along the body. Mouth, pharynx, and short oesophagus are lined by a chitinous cuticle, hke that of the exterior. The long endodermic digestive region or mid- gut extends from the second leg nearly to the end of the body. Its walls are plaited. Finally, there is a short rectum or proctodaeum, lined by a chitinous cuticle. Circulatory system. — The dorsal blood vessel forms a long con- tractile heart. It lies within a pericardial space, and receives blood by segmentally arranged apertures with valves. The circulation is mostly in ill-defined spaces in the apparent body cavity or " haemocoele." 3^4 PHYLUM ARTHROPODA Respiratory system. — Very long and fine unbranched tracheae are widely distributed in the body ; a number open together to the exterior in flask. like depressions. These openings or stigmata are irregularly distributed. Excretory system. — A pair of nephridia lie in each segment. Each consists of an internal mesodermic terminal funnel, a looped canal, and a wide vesicle which opens near the base of each leg, the two last parts being invaginations of the ectoderm. Nephridia are not known in any other Tracheate. The salivary glands and the genital ducts seem to be modified nephridia. It may be noted that the same is probably true of the " coxal glands " of Limulus and of the antennary glands of Crustaceans. Coxal or crural glands lie in the legs and open to the exterior. They can be in part evaginated, and they probably help in respiration. In the male of P. capensis the last pair are very long (Fig. 203, a.g.). The large mucous glands, which pour forth slime from the oral papillae, are regarded as modified crural glands. Reproductive system. — (a) Female (of P. edwardsii). — From the two ovaries, which are surrounded by one connective tissue sheath, and arise, as usual, from the coelomic epithelium, the ova pass by two long ducts leading to a common terminal vagina opening between the second last legs. These ducts are for the most part uteri, but on what may be called the oviduct portions adjoining the ovaries there are two pairs of pouches — a pair of receptacula seminis (for storing the spermatozoa received during copulation), and a pair of receptacula ovorum for storing fertilised eggs. The eggs are hatched in the uteri, and all stages are there to be found in regular order. The young embryos seem to be connected to the wall of the uterus by what has been called a " placenta," so suggestive is it of mammalian gestation. The older embryos lose this " placenta," but each lies constricted off from its neighbours. When born the young resemble the parents except in size and colour. In P. capensis the period of gestation is thirteen months. (6) Male (of P. edwardsii). — The male elements are produced in small testes, pass thence into two seminal vesicles, and onwards by two vasa deferentia into a long single ejaculatory duct, which opens in front Fig. 204. — Embryos of Peripatus capensis, showing closure of blastopore and curvature of embryo. — After Korschelt , and Heider. a., Anus ; bl., blastopore ; m., mouth ; p.s., primitive segments ; w., zone of proliferation. DEVELOPMENT OF PERIPATUS 365 of the anus. In the ejaculatory duct the spermatozoa, which are thread-like, are made into spermatophores which are attached to the female. It is uncertain how the spermatozoa get into the female. Fertilisation is ovarian. While it is characteristic of Arthropods, in which chitin is so pre- dominant, that ciliated epithelium is absent, it seems that in Peripatus, which is much less chitinous than the others, ciliated cells occur in some parts of the reproductive ducts. Development. — There is some variety of development in different species. Thus there is much yolk in the ovum of P. novcB zealandicB, extremely little in that of P. capensis. In P. capensis the " segmentation " is remarkable, for true cleavage of cells does not occur. The fully " segmented " ovum does not exhibit the usual cell limits. It is a protoplastic mass — or syncytium — with many nuclei. Even when the body is formed, the continuity of cells persists, nor does the adult lack traces of it. To Prof. Sedgwick this singular fact suggested the theory that the Metazoa may have begun as multinucleate Infusorian-like animals. The gut appears from a fusion of vacuoles within the multinucleated mass, and a gastrula stage is thus established. The blastopore or gastrula mouth closes except at the two ends, thus forming mouth and anus. In its early phases the development resembles in many ways that of an Annelid. But thereafter Arthropod characters super- vene. Paired rudiments of appendages grow out (see Fig. 204, lower right-hand figure), beginning with the antennae. The development of the coelom becomes largely suppressed in favour of a quite different set of cavities which become filled with blood and constitute the hcemocoele. In the ova of P. novce zealandice, which have much yolk, a superficial multiplication of nuclei forms a sort of blastoderm, which spreads over almost the entire ovum. The segmentation in this case has been called centrolecithal (the type characteristic of Arthropods), but it is again true that for a long time the cells do not exist as well-defined units. It has been said, indeed, that " the embryo is formed by a process of crystallising out in situ from a mass of yolk, among which is a proto- plasmic reticulum containing nuclei." Zoological position. — The synthetic characters of Peripatus and its allies may be thus summarised : — Annelid Characteristics. Segment ally arranged nephridia as in Chaetopods. The muscular ensheathing of the body. The cilia in the genital ducts. Less important are the stump-like hollow legs and the simple eyes. Arthropod and Tracheate Characteristics. The presence of tracheae. The nature of the heart (a tube with paired ostia communicat- ing with a pericardium) and the lacunar circulation. The modification of appendages as mouth organs. The form of the salivary glands. The smallness of the genuine coelom ; the cavity of the body is haemocoelic. 366 PHYLUM ARTHROPODA The Onychophora differ from other Tracheata Antennata in the simphcity and diffuseness of the tracheae, in having only one pair of jaws, in the absence of external segmentation, in the nature of the body wall, and so forth. The ladder-like character of the ventral nervous system (cf. primitive Molluscs, Ph^^llopod Crustaceans, and Nemerteans) is probably primi- tive. That salivary glands and genital ducts are homologous with nephridia is a fact of much morphological interest. It is possible that the slime glands are modifications of crural glands, and that the latter are homologous with the parapodial glands of some Annelids. It is Fig. 205. — A millipede. Fig. 206. — A centipede. not certain that the antennae, jaws, and oral papillae of Peripatus precisely correspond to the antennae, mandibles, and first maxillae of Insects. Qur general conclusion is that Peripatus is an archaic type, a sur- vivor of forms which were ancestral to Tracheata and closely related to Annelids. Like some other archaic types, e.g. Dipnoi, the Onychophora have a very wide range of distribution, which may be briefly indicated : — Peripatus (tropical America and tropical Africa) ; Eoperipatus (Indo- Malay) ; Peripatoides and Ooperipatus (Australasia) ; Opisthopaius (Chili and South Africa) ; Paraperipatus (New Britain) ; Peripatopsis (Central Africa). CENTIPEDES AND MILLIPEDES 367 Second Class of Tracheata Antennata.— Myriopoda. Centipedes and Millipedes The centipedes and millipedes, which are grouped together in the class Myriopoda, are usually elongated, somewhat vermiform animals, with a distinct head and a IVA Fig. 207. — Mouth-parts and poison claws of a centipede. — From a Specimen. I. Upper lip or labrum. II. Mandibles. III. First maxillae. IV. Palp (exopodite) of second maxilla. IVa. Inner portion (endopodite) of second maxilla. V. Poison claws or first pair of legs. VI. jpasal plate or sternal region of first trunk segment. very uniform segmented trunk. The head bears eyes (groups of eye-spots, not compound eyes like those of insects, except in Scutigera), jointed antennae, and two or three pairs of jaws. The segments of the trunk bear six- or seven- jointed legs with terminal claws, very similar through- out. "The nervous system, the tracheae, the heart, the ex- cretory tubules, etc., are like those of Insects. It cannot 368 PHYLUM ARTHROPODA be said that the centipedes (Chilopoda) and the miUipedes (Diplopoda) are very closely related to one another, and there are two other distinct orders, Symphyla and Pauro- poda. The resemblances are in part resemblances of I n.c Fig. 208. — Mouth-parts of a millipede. — From a Specimen. I. Mandibles. IIa. Outer blade (exopodite) of first maxilla. IIb. Inner blade (endopodite). lie. Basal piece or protopodite of first maxilla. convergence, not of genuine affinity. Simple wingless insects, known as Collembola and Thysanura, are closely approached by such Myriopods as Scolopendrella ; and it is likely that Myriopods and Insects are divergent branches from a common stock. Centipedes and millipedes are characteristically terrestrial . Most are very shy animals, lurking in dark places and avoiding the light, but it is interesting to note that at least two Myriopods — Geophilus submarimis and Linotcenia maritima — occur on British coasts. CENTIPEDES AND MILLIPEDES 369 MYRIOPODA Centipedes. Chilopoda. Carnivorous. Poisonous. Body usually flat. One pair of appendages to each segment. The stigmata do not correspond in number to the segments ; they often occur on alternate segments. Many-jointed antennae. Toothed cutting mandibles. Two pairs of maxillae, usually with palps. The first pair of legs modified as poison claws. A single genital aperture on the second last segment. Examples. - -Scolopendra. Lithohius. Geophilus. Millipedes. DiPLOPODA (or Chilognatha). Vegetarian. Harmless. Body cylindrical. By the imperfect separation of the segments, all but the first three behind the head seem to have two pairs of appendages each, and also two paired ganglia, and two pairs of stigmata (tracheal openings). Seven-jointed antennae. Broad masticating mandibles. A pair of maxilla fused in a broad plate, usually four-lobed. No poison claws. Genital apertures open an- teriorly. Examples. — Julus. Polyxenus. Glomeris. In the order Symphyla {Scolopendrella) there are not more than twelve segments, and there is only one pair of tracheae, which open on the head. Scolopendrella is in several ways hke the primitive insects known as Thysanura. In the order Pauropoda {Pauropus) there are ten segments, and the antenuce are branched. Third Class of Tracheata Antennata.— Insecta Insects occupy a position among the backboneless animals like that of birds among the Vertebrates. The typical members of both classes have wings and the power of true flight, richly aerated bodies, and highly developed respiratory, nervous, and sensory organs. Both are very active and brightly coloured. They show parallel differ- ences between the sexes, and great wealth of species within a narrow range. 24 370 PHYLUM ARTHROPODA General Characters Like other Arthropods^ Insects have segmented bodies, jointed legs, chitinous armature, and a ventral chain of ganglia linked to a dorsal brain. Compared zvith Peripatus and Myriopods, adult insects show concentration of the body segments, decrease in the number and increase in the quality of the appendages, and wings in the great majority. Insects are terrestrial and aerial, and rarely aquatic animals ; usually winged as adults, breathing by means of Fig. 209. — Female cockroach (P. orientalis). Fig. 209A. — Male cockroach (P. orientalis). trachece, and often with a metamorphosis in the course of their life-history. The body is divided into three distinct regions — head, thorax, and abdomen. The head bears a pair of pre-oral antennce, and three pairs of mouth appendages ; the thorax bears a pair of legs on each of its three segments, and, typi- cally, a pair of wings on each of the posterior two ; the abdomen has no appendages, unless these be represented by stings, ovipositors, etc., or else by vestiges. First Type of Insects, Periplaneta (or Blatta). — The Cockroach Habits. — The cockroaches in Britain are immigrants from the East (P. orientalis), or from America (P. americana). STRUCTURE OF COCKROACH 371 They are omnivorous in their diet, active in their habits, hiding during the day and feeding at night. They are mp Fig. 210. — Mouth -parts of cockroach. — From a Specimen. MN., Mandibles with iutenial cutting teeth. CA., Cardo of first maxilla, and ST., the stipes, the two forming the basal piece or protopodite. L., Lacinia, and G., the galea, the two forming the inner piece or endopodite. MX. P., The five-jointed maxillary palp, the exopodite. SM., The sub-mentum of the second maxilla or labium, and M., the mentum, the two forming the basal piece or protopodite. LA., Lacinia, and PG., the paraglossa, the two forming the inner piece or endo- podite (sometimes called the ligula). L.P., The three-jointed labial palp, the exopodite, rising from a little process or palpiger of the mentum. ancient insects, for related forms occurred in Silurian ages ; they are average types, neither very simple nor very highly 372 PHYLUM ARTHROPODA External Characters Region. The head is ver- tically elongated and separated from the thorax by a neck. The insect's head seems to consist of seven fused segments^ — ocular, an teniiary, intercalary, man- dibular, maxillu- lar.maxillary, and labial. Appendages. The thorax con- sists of three seg- ments — (a) prothorax, (b) mesothorax. (c) metathorax. (Each segment is bounded by a dorsal tergum and ventral ster- num.) The ' abdomen consists of lo (or ii) distinct segments, with terga and sterna as in the thorax. The first sternum is rudimentary in both sexes, and in the female the eighth and ninth segments are con- cealed by the large seventh. 1. The antenna (probably homologous with appendages), long, slender, many- jointed, tactile. 2. A pair of stout toothed mandibles working sideways. 3. The first maxilla;, each consisting — (a) of a basal piece or protopodite with two joints : a basal cardo, a distal stipes ; (b) of a double endopodite borne by the basal piece, and consisting of an inner lacinia and a softer outer galea ; (c) of an exopodite or maxillary palp also borne by the basal piece, and consist- ing of five joints. 4. The second pair of maxillae, fused to- gether as the " labiuin," consisting — (a) of a fused basal piece or protopodite with two joints : a basal sub-mentum, a smaller distal mentum ; on each side this protopodite bears^ (b) a double endopodite (ligula) consisting of an inner lacinia and an outer paraglossa ; (c) an exopodite or labial palp, consisting of three joints. (a) First pair of legs. (b) Second pair of legs. (c) Third pair of legs. Each leg consists of many joints — a basal expanded " coxa " with a small " trochanter " at its distal end, a " femur," a '' tibia," a six-jointed tarsus or foot ending in a pair of claws (Fig. 212). Two cigar-shaped tactile anal cerci, at- tached under the edges of the last tergum, are possibly relics of the last abdominal appendages. The ninth sternum of the male bears a pair of styles, possibly relics of appendages. Both sexes have complex hard structures (gonapophyses) beside the genital apertures. They are possibly relics of appendages. Other Structures. The large black compound eyes. The " upper lip " or labrum, in front of the mouth. The white oval patches near the bases of the antenna;, pos- sibly sensory. In some primitive insects a minute pair of appendages, known as maxillulae, occurs between the mandibles and the first maxillae. (b) A pair of wing-covers (modi- fied wings), rudimentary in female of P. orientalis. (f) A pair of membranous wings, sometimes used in flight, folded when not in use, absent in female of P. orientalis. Between the segments of the thorax are two pairs of respira- tory apertures or stigmata. A pair of stigmata occur be- tween the edges of the terga and sterna in the first eight abdo- minal segments. The anus is tenninal, beneath the tenth tergum of the abdo- men ; a pair of " podical plates " lie beside it. The genital aperture is on the eighth segment in the female, behind the niiath sternum in the male. The opening of the sperma- theca — the female's receptacle for spermatozoa — lies on the ninth sternum of the abdomen. I STRUCTURE OF COCKROACH 373 Specialised. Their position is among the Orthoptera, in the same order as locusts and grasshoppers. The hatched young are like miniatures of the adults, except that wings are absent. If there are wings, they appear at the last moult, when the cockroach becomes sexually mature. Skin. — There is an external chitinous cuticle and a subjacent cellular layer — the epidermis or hypodermis Fig. 211. — Ventral aspect of male cockroach with the wings extended. An imaginary median line has been inserted. A., Antennae; E., eye; P.T., pro thorax ; W^., first pair of wings; W^., second pair of wings ; C, cercus ; St., style ; Co., coxa ; Tr., trochanter ; F., femur ; Ti., tibia ; Ta., tarsus. — from which the cuticle is formed. The newly hatched cockroaches are white, the adults are dark brown. Moulting, which involves a casting of the cuticle, of the internal lining of the tracheae, etc., occurs some seven times before the cockroach attains in its fifth year to maturity. The muscles which move the appendages, and produce the abdominal movements essential to respiration, are markedly cross striped. They are in many cases attached to special tendons, which arise as cuticular invaginations, and are lost and replaced at each moult. 374 PHYLUM ARTHROPODA Nervous system. — A pair of supra-oesophageal or cere- bral ganglia lie united in the head. As a brain they receive impressions by antennary and optic nerves. By means of a paired commissure surrounding the gullet, they are con- nected with a double ventral chain of ten ganglia. Of these, the first or sub-oesophageal pair are large, and give off nerves to the mouth-parts, etc. ; from each of the ganglia of the thorax and the abdomen nerves are given off to adjacent parts. There are three pairs of ganglia in the thorax, and six in the abdomen, of which the last is the largest. From the oesophageal commissures visceral nerves are given off to the gullet, crop, and gizzard. Besides the large compound eyes, there are other sensory structures — -some of the setae on the skin, the maxillae (to some extent organs of taste), the antennae (tactile and olfactory), the anal cerci (tactile), and possibly the oval white patches on the head. Alimentary system. — (i) The fore-gut (stomodaeum) is lined by a chitinous cuticle continuous with that of the outer surface of the body. It includes — (a) the buccal or mouth cavity, in which there is a tongue-like ridge, and into which there opens the duct of the salivary glands ; (^) the narrow gullet or oesophagus ; (c) the swollen crop ; {d) the gizzard, with muscular walls, six hard cuticular teeth, and some bristly pads. Fig. 212. — Leg of cockroach. c, Broad expanded coxa; tr., troch- anter; /..femur; ti., tibia; /a., six- jointed tarsus with terminal claws and adhesive cushions. There is a pair of diffuse sahvary glands on each side of the crop, and between each pair of glands a salivary receptacle. The ducts of the two salivary glands on each side unite ; the two ducts thus formed combine in a median duct, and this unites with another median duct formed from the union of the ducts of the receptacles. The common duct opens into the mouth. (2) The mid-gut (mesenteron) is lined by endoderm. It is short and narrow, and with its anterior end seven or STRUCTURE OF COCKROACH 375 eight club-shaped digestive (pancreatic) outgrowths are connected. (3) The hind-gut (proctodaeum) is Hned by a chitinous cuticle. It is convoluted and divided into narrow^ ileum, wider colon, and dilated rectum with six internal ridges. Respiratory system.— The tracheal tubes, which have ten pairs of lateral apertures or stigmata, ramify throughout the body, and have a spirally thickened chitinous hning. Circulatory system. — The chambered heart lies along the mid-dorsal line of abdomen and thorax. It receives Fig. 213. — Transverse section of insect. — After Packard. h., Heart ; j?., gut ; n., uerve-cord ; St., stigma ; tr., trachea ; w., wing ; /., femur of leg. blood by lateral valvular apertures from the surrounding pericardial space, and drives it forwards by a slender aorta. The blood, perhaps better called haemolymph, circulates, however, within ill-defined spaces in the body. Excretory system. — There are sixty or so fine (Mal- pighian) tubules, which rise in six bundles from the begin- ning of the ileum, and twine through the " fatty body " and in the abdominal cavity. They are often found to be charged with excretory products (urates), apparently collected and passed into the lumen by the hning cells. The free end of each tubule is closed. The absence of nephridia in insects has been already mentioned. 376 PHYLUM ARTHROPODA Reproductive System Of the Male. Of the Female. The testes are paired organs, sur- rounded by the fatty body below the 5th and 6th ab- dominal terga. They atrophy in the adult. From the testes, two narrow ducts or vasa deferentia lead to two seminal vesicles. These seminal vesicles (the "mushroom-shaped gland") open into the top of the ejacu- latory duct. This duct opens between the 9th and loth sterna. Beside the aperture, there are copulatory structures (gonapophyses). With the ejaculatory duct a gland is associated. Large branched tubular glands secrete a volatile alkaline sub- stance, with a strong mousy odour, probably offensive to enemies. The ovaries are paired organs, in the posterior abdominal region, each consisting of eight ovarian tubes. These are bead-Hke strings of ova at various stages of ripeness. From the ovarian tubes of each side eight eggs pass at a time into a short wide oviduct. The two oviducts unite and open in a median aperture on the 8th abdominal sternum. Be- side the aperture are hard structures (gonapophyses) which help in the egg-laying. On the 9th abdominal sternite a pair of arborescent glands pour out their cementing secretion by two apertures. The sper- matheca is a paired sac opening between the 8th and the 9th abdominal sternum. Sixteen ova, one from each ovarian tube, are usually enclosed within each egg-capsule. The latter is formed from the partly calcareous secretion of the arborescent glands. Each egg is enclosed in an oval shell, in which there are several little holes (micropyles), through one of which a spermatozoon enters. Spermatozoa, from the store within the spermatheca, are included in the egg- capsule. At an early stage in development some cells associated with the mesoderm are set apart as reproductive cells, and originally these have a segmental arrangement as in Annelids ; at a later stage other meso- derm cells join these, some forming ova, others epithelial cells around the latter. The distinction between truly reproductive cells and THE HIVE-BEE 377 associated epithelial cells, which is said to be late of appearing in some of the higher insects, is established at a very early stage in the cockroach. Second Type of Insects. — The British Hive-Bee (Apis melUfica) This is a much more highly speciaUsed type than the cockroach. It belongs to the order Hymenoptera. Habits.— The Hive-Bee {Apis mellifica) is a native of this country, and is the species most commonly found domesticated. It is the only British representative of the genus ApiSy and exhibits, in its most fully developed form, the social life which is foreshadowed among the Humble- Bees. As a consequence of this social hfe, there is much division of labour, which expresses itself alike in habit and in structure. The males (drones) take no part in the work of the colony, and are wholly reproductive ; the females include the queen-bees and the workers. In the workers, which perform all the work of the hive, the reproductive organs are normally abortive and functionless. In the queens, of which there is but one adult to each hive, the enormous development of the reproductive organs seems to act as a check upon the brain and other organs, which are less developed than in the workers. The workers are further divisible into nurses, which are young and do not leave the hive, being occupied with the care of the larvae, and the older foraging bees, which gather food for the whole colony. In considering the relation between the life of the Hive- Bee and that of many allied forms {Bombus, etc.), it is important to notice that the habit of laying up stores of food material for the winter enables the colony, and not merely an individual, to survive, and must thus have greatly assisted in the evolution of sociality. External features. — The body shows the usual division into head, thorax, and abdomen, and varies considerably in the three different types, being smallest in the workers. It is entirely covered with hairs, some of which are sensitive, while others are used in pollen-gathering, etc. The head bears antennas, which are composed of a long basal and numerous smaller joints. They are marvellously sensitive, serving to communicate impressions, and also containing organs of special sense. A pair of compound eyes, largest in the drones, and three median ocelli, 378 PHYLUM ARTHROPODA are also present in the head region. Of the other appendages of the head, the mandibles are in the workers very powerful, and used for many purposes connected with comb-building. In the first maxillae the maxillary palps are aborted, and the appendage consists of an un- divided lamina at each side, borne on a basal piece consisting as usual of stipes and cardo. The second pair of maxillee form as usual the labium or so-called lower lip. and are much modified. The united basal joints form the mentum and sub-mentum. From the mentum at either side springs the long labial palp, which represents the outer fork of the typical appendage. The endopodite at each side is divided into two parts, but the inner two (laciniae) are united, much elongated, and form the tongue or ligula of the bee. The outer halves form the paraglossae, which are closely apposed to the base of the ligula. It is the great elonga- tion of the ligula and labial palps which especially fits the bee for nectar-gathering. The three structures can be closely apposed to one another, and then form an air-tight tube, up which, by the action of the stomach, nectar is sucked. In many of our British bees the ligula is much shorter, and more or less trowel-like in shape, and is then used largely, as in wasps, in the operation of plastering the nest. In such cases the bee can only suck those flowers in which the nectar is superficial. The hive- bees and humble-bees, on the other hand, are specially modi- fied to enable them to extract nectar from tubular flowers. When not in use, the elongated mouth-parts are folded back upon themselves, not coiled as in butterflies and moths, where there is even greater elongation. In the queen and in the drone the mouth -parts are shorter, and are not used in honey-gathering. pw ■ The thoracic appendages consist of cours? of three pairs of legs, which have the usual parts. On the first leg, at the junction of the tibia and the first tarsal joint, there is a complicated mechanism which is em- ployed in cleaning the antennae ; this is present in all three forms, and Fig. 2x4. — Head and mouth parts of bee. — After Cheshire. a.. Antenna ; m., mandible ; g., labruni or ' epipharj'nx ; mx.p., rudiment of maxil- lary palp ; mx., lamina of maxilla ; I. p., labial palp ; /., ligula ; b., bouton at end. The paraglossffi lie concealed between the basal portions of the labial palps and the ligula. THE HIVE-BEE 379 varies with the size of the antennae. In the workers the third leg is remarkably modified for pollen-gathering purposes. The first tarsal joint bears regular rows of stiff straight hairs on which the pollen grains are collected ; they are borne to the hive in the pollen basket, placed at the back of the tibia, and furnished with numerous hairs. In queen and drone these special arrangements of hairs are absent. The second and third thoracic segments bear each a pair of wings. These are largest in the drones and relatively smallest in the queen, who flies but seldom. At the base of each wing there is a respiratory spiracle. In the adult queen and worker, the abdomen is divided into six segments ; in the drone, into seven. There are no abdominal appen- dages. On the ventral surface in the worker, but not in the queen or drone, there are four pairs of wax pockets or glands, which secrete the wax, which, after mastication with saliva, is employed in building the combs. The abdomen also bears in queen and worker five pairs of spiracles, but in the drone, on account of the additional segment, there are six pairs. The total number of spiracles is thus fourteen for queen and worker, and sixteen for the drone. The posterior region of the abdomen bears the complicated sting. In the worker this consists of a hard incomplete sheath, which envelops two barbed darts. The poison flows down a channel lying between the darts and the sheath. Ramify- ing through the abdomen are found the two slender coiled tubes which constitute the poison gland. At the posterior end of the body these unite and open into a large poison sac. When a bee uses its sting, the chitinous sheath first pierces the skin, and then the wound is deepened by the barbed and pointed darts, while at the same time poison is steadily pumped down the channel mentioned above, and pours out by minute openings at the bases of the darts. The poison contains formic acid, and is fatal to the bee if directly introduced into its blood. Associated with the sting there are a pair of dehcate tactile palps. In the queen the sting is curved and more powerful, but it is apparently only used in combat with a rival. In the worker, the sting, and with it a portion of the gut, is usually lost after use, and, in consequence, death ensues ; the queen, on the other hand, can withdraw her sting from the wound with considerable ease. The sting is really an ovipositor adapted to a new function. Naturally, therefore, there is no trace of it in the drones. Nervous system. — In the adult this exhibits consider- able fusion of parts. The supra-6esophageal gangha are very large, and send large lateral extensions to the com- pound eyes. This " brain " is best developed in the active workers. The sub-oesophageal mass is formed by the fusion of three pairs of ganglia. In the thorax there are two pairs of ganglia, of which the second supplies the wings and the two last pairs of legs. In the worker there are five pairs of abdominal ganglia, but in the queen and drone only four. The sense organs are the simple and compound 38o PHYLUM ARTHROPODA eyes, and the antennae, which are furnished with numerous sensitive structures, in great part olfactory. Alimentary system. — The cESOphagus is a narrow tube which runs down the thoracic region. In the abdominal region it expands into the crop or honey-sac. The crop opens by a complicated orifice, with a remarkable stopper arrangement, into the digestive region or chyle stomach, Fig. 215. — Nervous system of bee. — After Cheshire. A. Of larva. B. Of adult, a., Antenna ; mx., maxilla ; m., mandible ; a'., origin of wing ; 1-5, abdominal ganglia. which is separated by a pylorus from the coiled small intestine. The inner wall of the small intestine bears numerous rows of chitinous teeth set in longitudinal ridges, and is perforated by the apertures of the excretory tubules. At the junction of the small with the large intestine there are six brownish plates, perhaps functioning as valves. In connection with the anterior region oi the gut there is a very complicated series of glands. First we have, in the workers only, on Fig. 2 1 6.— Food canal of bee.— In part after Cheshire. mx., MaxUla ; a., antenna ; e., eye ; s.g., salivary glands ; oe., CBSophagus ; h.s., honey-sac ; s., stopper ; c.s., chylific stomach : m.t., Malpighian tubules ; s.i., small intestine ; l.i., large intestine ; St., poison sac; stg., sting. 382 PliYLUM ARTHROPODA either side of the head, a long coiled gland which is intracellular in type. It is largest in the so-called " nurses " which feed the young, and diminishes in size later. According to Mr. Cheshire, this gland secretes a nitrogenous fluid which is furnished to all the larvae in their early stages, but is supplied to the future queen during the whole of the feeding period, and also during the period of egg-laying ; this secretion was formerly termed " royal jelly." It is this differential feeding which determines the appearance of the specialised queen ; in the earliest stages, queen and worker larva? are identical. In addition to this pair of glands, there are in the worker three other gland systems. Of these, the second and third pairs have a common central outlet on the mentum, and secrete the saliva, which is plentifully mixed with the nectar during suction. The fourth pair is small, and the ducts open just within the mandible. The last three pairs of glands are found also in drone and queen. The method of feeding in the bee differs considerably in the three types. In the worker, the nectar sucked up from flowers is mixed with sahva, passes down the gullet into the crop, thence by the opening of the " stomach mouth " it may reach the true stomach and so be digested, or may be carried in the crop to the hive, and there emptied into the cells by regurgitation. The carbohydrates of the nectar are altered by the bee's digestive enzymes, cane- sugar being split or " inverted " into glucose and fructose, while starch is converted into gummy dextrin. The pollen, which is frequently mixed with the honey, is separ- ated from the latter by means of the stomach mouth, and is digested. Before impregnation, the queen, like the worker, feeds on pollen and honey ; after it, she is always fed by the attendant workers. The drones, like the young workers, avail themselves of the general food-supply of the colony, and do not themselves collect honey. Other systems. — The respiratory system is represented by the ramifying tracheal tubes. They open to the ex- terior by the lateral spiracles, which can be completely closed. In connection with the tracheae there are large air-sacs. The circulatory system is in essentials the same as that of the cockroach. The blood contains a few nucleated amoeboid corpuscles. The excretory system consists of numerous fine Mal- pighian tubules which open into the small intestine. Reproductive system. — In the drone the reproductive REPRODUCTION IN BEES 383 organs consist of a pair of testes, each furnished with a narrow vas deferens, expanding at its distal end into a seminal vesicle. The seminal vesicles open into the ejacu- latory duct, and at their junction a large paired mucus gland opens. When maturity is reached, the testes diminish in size, while the spermatozoa accumulate in the terminal expanded part of the ejaculatory duct, and there become aggregated into a compact spermatophore. With the ter- FiG. 217. — Hive-bees and the cells in which they develop. D., Drone cells ; W., worker cells ; Q., queen cell, open and closed ; d., drone ; w., worker ; q., queen. minal portion of the male duct copulatory organs are associated. Mating takes place only once in the life of the queen, and is followed by the death of the drone. In the queen the large ovaries occupy considerable space in the abdominal region. As usual, each consists of numerous (100-150) ovarian tubes, containing ova in various stages of maturity. The ovarian tubes open into the right and left oviducts, which again unite to form the common oviduct. With the anterior portion of the common 384 PHYLUM ARTHROPODA duct the globular spermatheca is associated. In connection with it there is a gland corresponding to the mucus gland of the male. The oviduct terminates in a copulatory pouch. Previous to laying, the eggs are fertilised by sperms set free from the spermatheca. In the case of drone eggs, this liberation of sper- matozoa does not take place, and the eggs in consequence are partheno- genetic. Queens which have never mated, or which have exhausted their stock of male elements, habitually lay drone eggs, but those which are laying abundant fertilised eggs at times also lay unfertilised eggs. This withholding of spermatozoa is said to be " voluntary," and related to the needs of the colony, but the physiological reason is unknown. The workers possess female organs similar in type to those of the queen, but of an extremely rudimentary nature. The eggs are laid singly in the cells of the comb, at the rate of about two per minute, for weeks together. They are of the usual insect type. According to the size of the cell in which it is deposited, and the food with which it is furnished, the fertilised ovum develops into a worker or into a queen. The development takes place within the cell, and includes a complete metamorphosis. Classification of Insects I. Primitive wingless insects, Apterygota or Aptera, including Thysanura, e.g. Machilis, Campodea, Lepisma ; Collembola, Springtails, e.g. Podura, Smynthurus. II. Winged insects, Pterygota (in some degenerate forms the wings have been lost). A. With mouth-parts usually adapted throughout life for biting (Menognathous), with no metamorphosis (Ametabolic) or with incomplete metamorphosis (HemimetaboUc). e.g. Orthoptera (cockroach, locust, cricket, etc.) ; Corrodentia (Termites, bird-lice) ; Odonata (Dragon-flies) ; Ephemerida (May-fiies) ; and Dermaptera (Earwigs). B. With mouth-parts adapted in the main as suctorial organs (Menorhynchous), usually with no metamorphosis (Ametabolic). e.g. Rhynchota or Hemiptera, e.g. Phylloxera, aphides, coccus insects ; Cicadas ; bugs ; water-scor- pions, lice. C. With complete metamorphosis (Holometabolic), with mouth-parts always adapted for biting (Menognathous), or adapted at first for biting and afterwards for sucking (Metagnathous). e.g. Coleoptera (beetles) ; Diptera (two-winged flies) ; Lepidoptera (butterflies and moths) ; Hymen- optera (ants, bees, and wasps). APPENDAGES 385 General Notes on Insects The main characteristics of insects have already been described in the two types chosen, but we here revise them in general terms. Form. — The body of an adult insect may be divided into three distinct regions : — 1. The head, probably consisting of seven fused segments. 2. The median thorax, divided into pro-, meso-, and meta-thoracic segments, each with a pair of legs, the last two often with wmgs. 3. The abdomen, usually with ten to eleven segments, with never more than transformed traces of appendages. Within these limits there is great variety of form, e.g. the long dragon-fly with its large outspread wings, the compact cockchafer, the thin-waisted wasps and long-bodied butterflies, the house-fly and cricket, the large moths and beetles, and the almost invisible insect parasites. Appendages. — Insects feel their way, test food, and apparently communicate impressions to one another, by means of the antennae. Then follow the mandibles, first maxillae, and second maxillae, on the head ; the three pairs of legs on the thorax ; and sometimes vestiges of legs on the abdomen. It was a step of some importance in morphology when Savigny showed that the three pairs of appendages about the mouth are homologous with the other appendages, i.e. are masticatory legs. (i) Farthest forward lie two mandibles, the biting and cutting jaws. These are single-jointed, and thus differ from the organs of the same name in the crayfish, which bear a three-jointed palp in addition to the hard basal part. In those insects which suck and do not bite, e.g. adult butterflies, the mandibles are reduced. (2) Next in order is the first pair of maxilla. Each maxilla consists of a basal piece (protopodite), an inner fork (endopodite), and an outer fork (exopodite). The entomologists divide the protopodite into a lower joint, the cardo, and an upper, the stipes ; the endopodite into an internal lacinia and an external galea ; while the exopodite is called the maxillary palp. (3) The last pair of oral appendages or second maxillcB are partially fused, and form what is called the labium. The lower and upper joints of their fused protopodites are called submentum and mentum ; the endopodites on each side are double, as in the first maxillas, and consist of internal lacinia and external paraglossa ; the exopodites are called the labial palps. The three pairs of thoracic legs consist of many joints, are usually clawed and hairy at their tips, and differ greatly according to their uses, 25 386 PHYLUM ARTHROPODA as may be seen by comparing, for instance, the hairy feet by aid of which the fly runs up the smooth window-pane, the muscular Umbs of grasshoppers, the lank length of those which characterise " daddy-long- legs," the bees' legs with their pollen baskets, the oars of water -beetles. -I-' u 3 in O o w +-> u a ft o 00 y3 s ^ 1-1 o CO 'Ji l« O OJ >^ - f-i - . —I to .^ a) ■'^•^" 03 C! ■ " " tn Lj Co •ft^ 6 ji c a a < Wings. — These arise as flattened hollow sacs, which grow out from the two posterior segments of the thorax. They are moved by muscles, and traversed by " veins " or WINGS OF INSECTS 387 " nervures," which include air-tubes, nerves, and vessel- Uke continuations of the body cavity. Most insects have two pairs, but many sluggish females and parasites, like lice and fleas, have lost them. On the other hand, there is no reason to believe that the very simplest wingless insects, known as Collembola and Thysanura, ever had wings. There are many interesting differences in regard to wings in the various orders of Insects. Thus in beetles the front pair form wing- covers or elytra ; in the little bee parasites — Strepsiptera — they are twisted rudiments ; in flies the posterior pair are small knobbed stalks (halteres or balancers) ; in bees the wings on each side are hooked together. When the insect is at rest, the wings are usually folded neatly on the back ; but dragon- flies and others keep them expanded ; butterflies raise them like a single sail on the back ; moths keep them flat. Many wings bear small scales or hairs, and are often brightly coloured. It is well known that the colours also vary with sex, chmate, and surroundings. Most interesting are those cases in which the colours of an insect harmonise exactly with those of its habitat, or make it a mimetic copy of some more successfully protected neighbour. As to the origin of wings, it may be mentioned that in many cases they are of some use in respiration as well as in locomotion, and the theory seems plau- sible that wings were originally respira- tory outgrowths, which by and by became useful for aerial locomotion. New organs seem often to have arisen by the pre- dominance of some new function in organs which had some prior signiflcance. Moreover, we can fancy that an increase in respiratory efficiency brought about by the outgrowths in qffestion would quicken the whole life, and would tend to raise insects into the air, just as terrestrial insects can be made to frisk and jump when placed in a vessel with relatively more oxygen than there is in the atmosphere. Finally, we must note that the aquatic larvae of some insects, e.g. may-flies, have a series of respiratory outgrowths from the sides of the abdomen, the so-called " tracheal gills," which in origin and appearance are like young wings (Fig. 219). Insects excel in locomotion. " They walk, run, and jump with the quadrupeds ; they fly with the birds ; they Fig. 219. — Young may-fly or ephemerid. — After Eaton. Showing tracheal gills, and Avings appearing in front of them. 388 PHYLUM ARTHROPODA glide with the serpents, and they swim with the fish." They beat the elastic air with their wings, and though there cannot be so much complexity of movement as in birds where the individual feathers move, the insect wing is no rigid plate, and its up-and-down motions are complex. They can soar rapidly, but their lightness often makes horizontal steering difficult. The wind often helps as well as hinders them ; thus the insects which fly in and out of the windows of express trains are probably in part sucked along. Marey calculates the approximate number of wing strokes per second at 330 for the fly, 240 for the humble-bee, 190 for the hive-bee, no for the wasp, 28 for the dragon-fly, 9 for a butterfly. For short distances a bee can outfly a pigeon. Skin. — As in other Arthropods, the epidermis (or hypo- dermis) of Insects forms a firm cuticle of chitin, which in the exigencies of growth has sometimes to be moulted. This cuticle is often finely marked, so that the animal seems iridescent ; and there are many diff"erent kinds of scales, hairs, and spines. Chitin is not favourable to the develop- ment of skin glands. Most insects have '* salivary glands " opening in or near the mouth. Bees have wax-making glands opening on the abdomen ; aphides have glandular tubes ; not a few have poison bags ; and many larvae besides silkworms have organs from which are exuded the threads of which a cocoon is made. Muscular system. — In very active animals like Insects, we of course find a highly developed set of rapidly contract- ing striped muscles. These work the wings, the legs, and the jaws. The resulting movements have this further significance, that they help in the respiratory interchange of gases, and in the circulation of the blood. Nervous system. — As in other Arthropods, the nervous system consists — (a) of a dorsal brain or supra-cesophageal ganglionic mass, and {b) of a double ventral nerve-cord with a number of paired ganglia, of which the most anterior (the sub-cesophageal) are linked to the brain by a ring com- missure around the gullet ; and (c) of nerves given off from the various ganglia to the sense organs, the alimentary canal, and the other organs. In many of the higher in- sects the ganglia of the ventral nerve-cord are in some SENSES OF INSECTS 389 degree concentrated, and in the adults are usually more centralised than in the larvae. Sensory structures. — Animals so much alive as Insects, and in surroundings so stimulating as many of them enjoy, have naturally highly developed sense organs. Two compound eyes are present on the head of all adults except the primitive CoUembola, the degenerate lice, the likewise parasitic fleas, and bhnd insects which live in caves or other dark places. Each eye contains a large number of similar elements, in each of which we can distinguish — (i) a cuticular or corneal facet ; (2) a glassy lens-like portion ; (3) a retinal portion in association with which are fibres from the optic nerve ; and there are also pig- mented cells between the elements. In addition to the compound eyes, simple eyes or oceUi are present in the adults of many insects, e.g. ants, bees, and wasps ; they occur without the accompaniment of com- pound eyes in CoUembola, lice, and fleas, and they are usually the only eyes possessed by larvae. They have only one lens (monomeniscous), whereas the compound forms have many lenses (polymeniscous). In the simple eye each retinal unit is a single cell, of which the distal part is un- pigmented. In the compound eye the retinal unit consists of six cells around an axis. The structure of ocelli varies greatly, and their use is very uncertain. Auditory (or chordotonal) organs have been found in all orders of Insects (except as yet the Thysanoptera), and occur both in the larvae and in the adults. Thek essential structure is as follows : — A nerve ends in a centre or ganglion near the skin ; some of the cells of this ganglion grow out into long sensitive rods enclosed in a tiny sheath ; the rods are directly or indirectly connected with the epidermis above them. " They are found in groups of 2-200 in various parts of the body, antennae, palps, legs, wings, in the halteres of Diptera„and upon the dorsal aspect of the abdomen." Quite different from these, and occurring in flies alone, on the hind end of the larva, or at the base of the adult's feelers, are httle bags with fluid in which clear globules float. In addition to the " eyes " and " ears," there are innervated hairs (tactile, tasting, olfactory) on the antennae and mouth-parts of many insects. Not a few insects seem to possess a diffuse or dermatoptic sense, by which, for instance, they can, when blinded, find their way out of a dark box. Many Insects produce sounds. We hear the whirr of rapidly moving wings in flies ; the buzz of leaf-like structures near the openings of the air-tubes in many Hymenoptera ; the scraping of legs against wing ribs 390 PHYLUM ARTHROPODA in grasshoppers ; the chirping of male crickets, which rub one wing against its neighbour ; the piping of male Cicadas, which have a complex musical instrument ; the voice of the death's-head moth, which expels air forcibly from its mouth. The death-watch taps with its head on wooden objects, as if knocking at the door behind which his mate may be hidden. In some cases the sounds are simply auto- matic reflexes of activity ; in many cases they serve as alluring love calls ; and they may also serve as expressions of fear and anger, or as warning alarms. In the case of hive-bees there is evidence of a power of "homing." They return straight to the hive from a distance of over a mile, even when they have been carried afield in a box. In great measure they learn the features of their district. Alimentary system. — The diet of Insects is very varied. Some, such as locusts, are vegetarian, and destroy our crops ; others are carnivorous (we need not specify the homceopathist's leech), and suck the blood of living victims, or devour the dead ; the bees flit in search of nectar from flower to flower, while the ant-lion lurks in his pit of sand for any unwary stumbler ; the terinites gnaw decaying wood ; some ants keep aphides as cows (" vaccae formi- carum," Linnaeus called them), whose sweet juices they lick ; and a great number of larvae devour the flesh and vegetables in which they are hatched. Many modifications of mouth organs, and of the alimen- tary canal, are associated with the way in which the insect feeds. The alimentary canal consists of fore-gut, mid-gut, and hind-gut, but in many cases it seems very doubtful if the mid-gut has its typically endodermic character. The fore-gut conducts food, and includes mouth cavity, pharynx, and cesophagus, the latter being often swollen into a storing crop, or continued into a muscular gizzard with grinding plates of chitin. The mid-gut is digestive and absorptive, often bear- ing a number of glandular outgrowths or caeca, and varies in length (in beetles at least) in inverse pro- portion to the nutritive and digestible quality of the food. The hind-gut is said to be partly absorptive, but is chiefly a conducting intestine, often coiled and terminally expanded into a rectum with which glands are frequently associated. RESPIRATION IN INSECTS 39 1 In association with the alimentary canal are various glands :-- (a) The salivary glands, which open in or near the mouth. They are usually paired on each side, and provided with a reservour. They arise as invaginations of the ectoderm near the mouth. Their secretion is mainly diastatic in fimction, i.e. it changes starchy material into sugar by means of a ferment. Along with these may be ranked the " spinning glands " of caterpillars, etc., which also open at the mouth. They secrete material which hardens into the threads used for the cocoon. (6) From the beginning of the mid-gut bhnd outgrowths sometimes arise (in some Orthoptera, etc.), which are apparently digestive. They are sometimes called pyloric caeca. In other cases (some beetles) there may be more numerous and smaller glandular outgrowths resembling villi in appearance. In the wood-eating termites, and in certain beetles, the hind-gut is crowded with symbiont Infusorians ; these apparently attack the insoluble substances of the wood and form simpler compounds available to the host as food ; without the Protozoa, termites starve on a wood diet. In the larva of the beetle Potosia, which lives in ants' nests and feeds on pine-needles, there are hosts of bacteria which dissolve and ferment cellulose. In other beetle larvae {Anohium) yeasts live in the cells lining the gut and must be supposed to aid in nutrition by supplementing the digestive enzymes of the host. Very many insects, especially those which suck blood or plant-juices, have their bacterial symbionts. Respiratory system.— The body of an insect is traversed by a system of air-tubes (tracheae), which open laterally by special apertures (stigmata), and by means of numerous branches conduct the air to all the recesses of the tissues. In animals which breathe by gills or lungs the blood is carried to the air ; in insects the air permeates the whole body. But how does the air pass in and out ? In part, no doubt, there is a slow diffusion ; in part the movements of the wings and legs will help ; but there are also special expiratory muscles. We see their action when we watch a drone-fly panting on a flower. Inspiration is passive, as in birds, and depends on the elasticity of the skin and of the tracheal walls ; expiration is active, and depends upon these muscles. They are chiefly situated in the abdomen, but in some beetles (at least) they are also present in the meta- thorax. 392 PHYLUM ARTHROPODA The tracheae seem to arise as tubular ingrowths of skin, and, primitively, each segment probably contained a dis- tinct pair ; but their number has been reduced, and they are often in part connected into a system. With the doubtful exception of one of the primitive CoUembola, and the certain exception of caterpillars, no insects have any tracheal openings in the head region. There are rarely more than two pairs in the thorax ; there are often six to eight pairs in the abdomen ; the maximum total is ten pairs. Each trachea is kept tense throughout the greater part of its course by internal chitinous thickenings, which apparently have a spiral course. The branches of the tracheae penetrate into all the organs of the body, carrying oxygen to every part. The very efficient respiration of insects must be kept in mind in an appreciation of the general activity of their life. As the conditions of larval life are often different from those of the adult insects, the mode of respiration may also differ in details. In insects without marked metamorphosis, and even in some beetles in which the metamorphosis is complete, the young insect and the adult both breathe by tracheae with open stigmata. Both are said to be " holopneustic." When the larvae live in water, the tracheal system is closed, other- wise the creatures would drown. This closed condition is termed " apneustic." These larvae (of dragon-flies, may-flies, and some others) breathe by " tracheal gills " (see Fig. 219) — little wing-like outgrowths from the sides of the abdomen, rich in tracheae — or by tracheal folds within the rectum, in and out of which water flows. In cither case, an interchange of gases between the tracheae and the water takes place. In adult aerial life the trachea of the body acquire stigmata, and the insect becomes " holopneustic." In most insects with complete metamorphosis, the larva {e.g. cater- pillar or grub) has closed stigmata on the last two segments of the thorax (those which will bear wings), but there is a pair of open stigmata on the prothorax. In the adult the reverse is the case. There are some other modifications — for instance, what obtains in the parasitic larvae of some flies, e.g. gadflies. In these the stigmata are open only at the end of the body. In all cases, however, the stigmata of the adult are already present as rudiments in the larva, though they may not open till adolescence is over. Circulatory system. — As the respiratory system is very efficient, air passing into the inmost recesses of the body, it is natural that the blood-vascular system should not be highly developed. Within a dorsal part of the body cavity, known as the pericardium, the heart lies, swayed by special EXCRETION IN INSECTS 393 muscles. It is a long tube, usually confined to the ab- domen, and with eight chambers, with paired valvular openings on its sides, through which blood or haemolymph enters from the pericardium. The blood is driven for- wards, the posterior end of the heart being closed, and there is usually an anterior aorta. But, for the most part, the blood circulates in spaces within what is commonly called the body cavity. Such a circulation is often de- scribed as lacunar. The blood may be colourless, yellow, red, or even greenish, and, in some cases, haemoglobin, the characteristic blood pigment of Vertebrates, has been detected. The cells of the blood are amoeboid. Sugars and amino-acids are present in the blood, in which the respiratory function seems to be unimportant in comparison with the distribution of food-materials. Body cavity. — It is necessary to distinguish the primitive ccelom from the apparent body cavity of the adult. Sedgwick notes the following characteristics of a true coelom : — It is a cavity which — (i) does not communicate with the vascular system; (2) does com- municate by nephridial pores with the exterior ; (3) has the repro- ductive elements developed on its hning ; (4) develops either as one or more diverticula from the primitive enteron (or gut), or as a space or spaces in the unsegmented or segmented mesoderm. Now, in Arthro- pods the apparent body cavity of the adult is not a true coelom : it consists of a set of secondarily derived vascular spaces ; • it has been called a pseudocode or a haemocoele. The true coelom of Arthropods is very much restricted in the adult. The apparent body cavity in which the organs lie, and in which the blood circulates, is well developed in Insects. It includes, intef alia, a peculiar fatty tissue, which seems to be a store of reserve material, which is especially large in young insects before metamorphosis, and is also interesting as one of the seats of " phosphorescence." Excretory system. — Although no structures certainly homologous with nephridia have been demonstrated in Insects, the excretory system is well developed. From the hind-gut (proctodaeum), and therefore of ectodermic origin, arise fine tubes, or in some cases solid threads, which extend into the apparent body cavity. Their number varies from two (in some Lepidoptera, for instance) to one hundred and fifty (in the bee). They twine about the organs in the abdominal cavity, and their excretory signifi- cance is inferred from the fact that they contain uric acid. Reproductive system. — Among Insects the sexes are always separate and often different in appearance. The 394 PHYLUM ARTHROPODA males are more active, smaller, and more brightly coloured than the females. Darwin referred the greater decorative- ness of the males to the sexual selection exercised by the females. The handsomer variations succeeded in courtship better than their rivals. Wallace referred the greater plain- ness of females to the elimination of the disadvantageously conspicuous in the course of natural selection. There may be truth in both views, but both require to be supplemented by the consideration, in part accepted by Wallace, that the " secondary sexual characters " of both sexes are the natural and necessary expressions of their respectively dominant constitutions. The organs consist of : — Male. Female. The paired testes, usually formed of many small tubes. Two ducts (vasa deferentia) con- ducting spermatozoa (perhaps in part comparable to neph- ridia). An unpaired terminal andejacula- tory duct, paired and with two apertures in Ephemerids only ; sometimes formed by a union of the vasa deferentia, sometimes by an external invagination meeting the vasa deferentia. From the vasa deferentia, or from the ejaculatory duct, opens a paired or unpaired seminal vesicle for spermatozoa. Various accessory glands, whose secretion sometimes unites the spermatozoa into packets or spermatophores. Sometimes a copulatory penis. Often external hard pieces. The paired ovaries, usually formed of many small tubes (ovarioles). Two ducts (oviducts) conducting the ova (perhaps in part com- parable to nephridia). An unpaired terminal region or vagina, paired and with two apertures in Ephemerids ; usually formed from an ex- ternal invagination meeting the united ends of the oviducts. Near or from the vagina, opens a receptaculum seminis for storing spermatozoa received from a male during copulation. Various accessory glands, t^./;:.those which secrete the material sur- rounding the eggs. Sometimes a special bursa copula- trix in the vagina. Often external hard pieces, e.g. ovipositor. Some peculiarities in reproduction. — Many Insects, such as aphides, silk-moth, and queen-bee, are exceedingly prolific? The queen termite lays thousands of eggs, " at the rate of about sixty per minute " ! DEVELOPMENT OF INSECTS 395 The store of spermatozoa received by the female, and kept within the receptaculum seminis, often lasts for a long time — for two or three years in some queen-bees. Parthenogenesis, or the development of ova which are unfertilised, occurs normally, for a variable number of generations, in two Lepidop- tera and one beetle, in some coccus insects and aphides, and in certain saw-flies and gall-wasps. It occurs casually in the silk-moth and several other Lepidoptera, seasonally in aphides, in larval life in some flies {Miastor, Chironomus), and partially or " voluntarily " when the queen- bee lays eggs which become drones. A few insects hatch their young within the body, or are " viviparous." This is the case with parthenogenetic summer aphides, a few flies, the little bee parasites Strepsiptera, a few beetles and cockroaches. Development of the ovum.— The tubes which com- pose the ovaries and lead into the oviducts begin as thin filaments, the ends of which are usually connected on each side. These thin filaments consist of indifferent germinal cells, all of them potential ova, and of mesodermic epithelial cells, which form the ovarian tubes, etc., and are connected anteriorly to the pericardial wall. But in most cases only a minority of these cells be- come ova, the others become nutritive cells which are absorbed by the ova, and follicle cells which line the walls of the ovarian tubes and help to furnish the egg- shells. There may be, indeed, ovarian tubes without nutritive cells {e.g. in Orthoptera), and then each tube is simply a bead-like row of ova, which become larger and larger as they recede from the thin terminal filaments and ap- proach the oviducts. In other cases the bead-like row consists of ova alternating with clumps of nutritive cells {e.g. in Hymenoptera and Lepidoptera). In other cases the nutritive cells mostly remain in the terminal region, but their products pass down to the receding ova. As there are numerous ovarian tubes in each ovary, and as the same process of oogenesis is going on in each, numerous eggs are ready for liberation at the same time, and are simultaneously discharged into the oviduct of each side. The eggs are relatively large and contain much yolk. In a few cases yolk is almost absent, as, for example, in the summer eggs of the Aphides, which are hatched within the body, and in some forms where the young are endoparasitic. 396 PHYLUM ARTHROPODA The ovum is surrounded by a vitelline membrane, and also by a firm chitinous shell, secreted by the follicular cells, which is often sculptured in a characteristic manner. This shell is pierced by one or more minute holes (micropyles). Through a micropyle the spermatozoon finds entrance, sometimes (as in the cockroach) after moving round and round the shell in varying orbits. The ripe egg usually consists of a central yolk-containing mass, sur- rounded by a thin sheath of protoplasm. As is usual in Arthropods, the segmentation is peripheral or centrolecithal. The central nucleus divides up into several nuclei, which, being united by protoplasmic cords, form for a time a central syncytium. Later, these nuclei emigrate into the peripheral protoplasm, which segments around them ; thus a peripheral layer of similar epithelial cells is formed. Some of the nuclei may be left behind in the central yolk to form the yolk nuclei, or, what is probably the more primitive condition, these are formed by subse- quent immigration from the blastoderm. The next process is the appearance of differentiation among the similar cells of the blastoderm. Over a special area — the ventral plate— (cf. Astacus) the cells increase in number and become cylindrical in shape ; over the rest of the egg the cells flatten out and become much thinner. In the middle of the ventral plate a slight groove is formed by rapid multiplication of the cyUndrical cells. This represents the disguised gastrulation, the open roof of the groove being the much-elongated blastopore. The surrounding cylindrical cells unite over this open roof, the groove usually flattens out, and thus we have formed a two-layered germinal streak which spreads forwards and backwards over the egg, and early exhibits externally transverse division into segments. The upper layer is the ectoderm ; the lower includes the rudiments of both mesoderm and endoderm. Meanwhile another very important event has taken place. We saw that while the cells of the ventral plate increased in depth, the remain- ing cells flattened out laterally ; at the point where the two kinds of cells unite, on either side of the ventral plate, a double fold arises. The two folds unite over the surface of the ventral plate, forming a mem- branous arch over it. The internal fold is called " amniotic," the outer " serous," from their resemblance to the similar envelopes in the embryos of higher vertebrates. The folds take no direct part in the development of the embryo. We must now return to the germinal streak. The gastrula groove may persist as a tube after closure of the blastopore, but it is usually compressed by the ectoderm, or never exists as a distinct cavity. The greater part of the lower stratum of the germinal streak consists of mesoderm. This becomes divided into successive segments at each side, each containing a primitive coelomic cavity, perhaps continuous with the gastrula cavity. The endoderm arises as paired clusters of cells, found only at the anterior and posterior ends of the primitive streak. These clusters increase rapidly and form long endodermal streaks, which curve downwards so as to enclose the yolk. The streaks LIFE-HISTORY OF INSECTS 397 meet and fuse, first ventrally and later dorsally, thus constituting the mid-gut. The yolk nuclei previously mentioned have meanwhile increased rapidly, forming yolk cells which absorb the yolk. These cells are included in the endodermic mid-gut, and there break up. As the endoderm grows round the yolk, it is accompanied by a layer (splanchnic) of the mesoderm. Fore- and hind-gut are formed by invaginations which fuse with the mid-gut. In the last stages of development the primitive ccelomic pouches lose their cross partitions, become filled with mesenchyme cells, and practically obliterated. The body cavity of the adult is formed by the appearance of lacunae amid the cells of the mesenchyme. The tracheae arise as segmentally repeated invaginations of the ecto- derm. The openings of the invaginations form the stigmata. From the hind-gut arise the Malpighian tubules, which are therefore ecto- dermic. The development of the other organs is similar to that of the Crustacea. In summarising the development of Insecta, one must specially note the peripheral segmentation, the formation of the two-layered germinal streak, the presence of an over- arching blastodermic fold, the segmentation of the meso- derm, and the formation of the mid-gut by the union of endodermic bands. Life-history of Insects. — (i) In the lowest Insects, namely, in the old-fashioned wingless Thysanura and Collembola, the hatched young are miniatures of the adults. By gradual growth, and after several moultings, they attain adult size. Similarly, the newly hatched earwigs, young of cock- roaches and locusts, of hce, aphides, termites, and bugs, are very like the parents, except that they are sexually im- mature, and that there are no wings, which indeed are absent from some of the adults. These insects are called ametabolic, i.e. they have no marked change or metamorphosis. (2) In cicadas there are slight but most instructive differences between larvae and adults. The adults live among herbage, the young on the ground, and the diversity of habit has associated differences of structure, as in the burrowing fore-legs of the larva. Moreover, the larva acquires the characters of an adult after a quiescent period of pupation. The differences between larva and adult are more striking in may-flies, dragon-flies, and the related Plecoptera {e.g. Per la) y for in these the larvae are aquatic, with closed Fig. 220. — Life-histories of Insects. L., P., and .4., larva, pupa, and adult respectively of water-beetle (Dytisciis marginalis) ; /., p., a., larva, pupa, and adult of blue-bottle fly {Musca vomitoria): I A, pA, a.i, larva, pupa, and adult of Cossus ligniperda. LIFE-HISTORIES OF INSECTS 399 respiratory apertures, and with tracheal gills or folds, while the adults are winged and aerial, and breathe by open tracheae. These insects are called hemimetaboUc , i.e. they have a partial or incomplete metamorphosis. (3) Very different is the life-history of all other sets of Insects— ant-lions, caddis-flies, flies, fleas, butterflies and moths, beetles, ants, and bees. From the egg there is hatched a larva (maggot, grub, or caterpillar), which lives a life very different from the adult, and is altogether unlike it in form. The larva feeds voraciously, grows, rests, and Fig. 221. — Life-history of the silk-moth (Bombyx mori). A., Caterpillar ; B, pupa ; C, imago ; the cocoon is cut open to show the pupa lying within. In the caterpillar note the three pairs of true legs in the anterior region, and the five pairs- of pro-legs in the posterior region. moults. Having accumulated a rich store of reserve material in its " fatty body," it finally becomes for some time quiescent, as a pupa, nymph, or chrysalis, often within the shelter of a cocoon. During this period there are great transformations ; wings bud out, appendages of the adult pattern are formed, reconstruction of other organs is effected. Finally, out of the pupal husk emerges a minia- ture winged insect of the adult or imago type. These insects are called holometabolic, i.e. they exhibit a complete metamorphosis. Two kinds of larvae occur among insects, (a) In many 400 PHYLUM ARTHROPODA ametabolic and hemimetabolic forms the larva is somewhat Hke one of the lowly Thysanuran insects {Campodea)^ and is therefore called campodeiform. It has the regions of the body well defined, three pairs of locomotor thoracic limbs, and biting or sucking mouth-parts, {b) The other type is worm-like or cruciform, e.g. the caterpillars of Lepidop- FiG. 222. — A typical caterpillar. — From a Specimen. A., The small antennse ; E., the simple eyes ; /., future prothoracic segment, with first stigma and first pair of jointed clawed legs ; //. and ///., second and third pairs of jointed clawed legs ; i, first pro-leg on the segment corresponding to the third abdominal ; ST., seventh stigma on the segment corresponding to the seventh abdominal ; 8, the eighth stigma ; H., a dorsal " horn " on the eighth abdominal segment ; 5, the fifth pro-leg on the tenth abdominal segment, the ninth being telescoped. In reality, at this stage, there are only two regions in the body — the head and the trunk ; but the first three segments after the head correspond to the future thorax, and the subsequent ten to the future abdomen. tera (Fig. 222), with distinct head and limbs ; the more modified grubs of bees, etc., with distinct head, but without limbs ; and the degenerate maggots of flies (Fig. 223, A), etc., not only limbless, but with an ill-defined head, A typical caterpillar has a cylindrical body often " hairy," a distinct hard head, simple eyes, small antennae, mouth- parts suited for biting, three pairs of jointed clawed thoracic limbs (corresponding to those of the butterfly), and four or Insect metamorphosis 461 five pairs of unjointed clasping abdominal " pro-legs." Other abdominal appendages are known on the larvae of other insects, and even in the embryos of some whose larvae are campodeiform. These facts make it likely that the primitive form had many legs. The larvce of Insects vary enormously in habit and in structure, and exhibit numerous adaptations to conditions of hfe very different from those of the parent. Thus caterpillars, which are usually plump and Fig. 223. — Development of blow-fly {CaUiphora erythrocephala). — After Thompson Lowne. The lower figure (A) shows the full-grown larva (maggot). Note, compared with the caterpillar, the absence of appendages, except those about the mouth ; h., the large hooks connected with the maxillae ; pi., pro-legs. The upper figure (B) shows the pronymph removed from the pupa- case. In the abdominal region the imaginal discs are shown ; /., rudiments of legs ; w., of wings. tense, so that a peck from a bird's bill may cause them to bleed to death, even if no immediate destruction befall them, are protectively adapted in many different ways. Their colours arexjften changed in harmony with those of their surroundings , some palatable forms are saved by their superficial resemblance to those which are nauseous ; a few strike " terrifying attitudes " ; while others are like pieces of plants. Internal metamorphosis. — In Insects with no marked metamorphosis, or with merely an incomplete one, the organs of the larvae develop gradually into those of the adult. But in Insects with complete metamorphosis there is a marvellous internal reconstruction during the later 26 402 PHYLUM ARTHROPODA larval, and especially during the quiescent pupal stage. The more specialised larval organs are disrupted, their debris being used in building new structures. In some cases, such as flies, phagocytes play a very important part in this metamorphosis ; in other cases there is no true phagocytosis. Parts of larval organs which have not been highly specialised form the foundations of new adult structures. Of special im- portance are certain ingrowths jv\ P of the larval skin (the epi- or hypo- dermis) which form w^hat are called *' imaginal discs," i.e. embryonic or ger- minal areas, from which arise the wings, legs, etc., of the adult insect. The reconstruc- tion is very thorough ; most of the musculature, much of the tracheal system, part of the mid-gut, etc., are gradually re- placed by the corresponding organs of the adult. There is first a disruptive process of histolysis, and then a recon- structive process of histo- genesis. Yet in most cases the Fig. 224.— Fly about to emerge disruption and replacement of from pupa-case, one end of organs is very gradual. ■ Ite Ha'^^r'' ^'''^^'' ''^'~ Ecology.— The average in- ^\ / * ,, „ sect is active, but between E., Compound eyes; M.P., mouth- , / ' , , parts; £1., first leg ; P.C., pupa- ordcrs {e.g. auts, bccs, and case; S., respiratory aperture into ^aspS VeVSUS aphidcS, COCCUS pupa-case. . jt 1 1 \ 1 insects, and bugs), between nearly related families, between the sexes {e.g. male and female cochineal insect), between caterpillar and pupa, we read the constantly recurrent antithesis between activity and passivity. The average length of life is short. Queen-bees of five years, queen-ants aged thirteen, are rare exceptions. In ECOLOGY OF INSECTS 403 many cases death follows as the rapid nemesis of repro- duction. But though the adult life is often very short, the total life may be of considerable length, as in some Ephemerids, which in their adult life of winged love- making may be literally the flies of a day, while their aquatic larval stages may have lived for two years or more. The relation between the annual appearance of certain insects and that of the plants which they visit, the habits of hibernation in the adult or larval state, the occasional " dimorphism " between winter and summer broods of butterflies, should be noticed. The prolific multiplication of many insects may lead to local and periodic increase in their numbers, but great increase is limited by the food-supply and the weather, by the warfare between insects of different kinds, by the numerous insects parasitic on others, by the appetite of higher animals — fishes, frogs, ant-eaters, insectivores, and, above all, birds. There is a great variety of protective adaptation. The young of caddis-flies are partially masked by their external cases of pebbles and fragments of stem ; many cater- pillars and adult insects harmonise with the colour of their environment ; leaf-insects, " walking sticks," moss-in- sects, scale-insects, have a precise resemblance to external objects which must often save them ; a humming-bird moth may resemble a humming-bird ; many palatable insects and larvae have a mimetic resemblance to others which are nauseous or otherwise little likely to be meddled with. Many insects may be saved by their hard chitinous armour, by their disgusting odour or taste, by their de- terrent discharges of repulsive formic acid, etc., by simula- tion of death, by active resistance with effective weapons. Many flowers depend for cross-fertilisation upon in- sects, which carry the pollen from one to another. Many insects depend for food on the nectar and pollen of flowers. Thus many flowers and insects are mutually dependent. But many insects injure plants, and many plants exhibit structures which tend to save them from attack. On the other hand, there may be " partnerships " between insects and plants — as in the " myrmecophilous " (ant-loving) plants, which shelter a bodyguard of ants, by whom they 404 PHYLUM ARTHROPODA are saved from unwelcome visitors. And again, the formation of galls by some insects which lay their eggs in plants, and the insect-catching proclivities of some carnivorous plants, should be remembered. Most insects are terrestrial and aerial ; the majority live in warm and temperate countries, but they are repre- sented almost everywhere, even above the snow-line, in arctic regions, in caves. Even on the sea the Challenger explorers found the pelagic Halohates, a genus of bugs. The distribution of insects is mainly limited by food- supplies and climate, for their powers fof flight are often great, and their opportunities of passive dispersal by the wind, floating logs, etc., are by no means slight. Many insects are more or less parasitic, either externally as adults, e.g. fleas, lice, bird-lice, plant-Hce, etc., or internally as larvae, e.g. the maggots of bot-flies in sheep, and a great number of borers within plants. We need only mention Hessian-fly, phylloxera, Colorado beetle, weevils, locusts, to suggest many more which Fig. 225. — Anurida are of much economic importance as maritima (after injurious insects. On the other hand, imms), one oy^e ^^j. indebtedness to hive-bee and silk- Colkmbol™* ^^^ moth, to cochineal and lac insects, to those which destroy injurious insects, and to those which carry pollen from flower to flower, is obvious. Finally, we must at least mention that in ants, bees, wasps, and termites we find illustration of various grades of social life, and marvellous exhibitions of instinctive skill as well as some intelligence. Pedigree. — Insects must have appeared relatively early, for remains of a cockroach-like form have been found even in Silurian strata. The higher forms with complete metamorphosis appear much later {e.g. beetles in the Carboniferous ages) ; but it seems that the Palaeozoic insects were mostly generalised types, prophetic of rather than referable to the modern orders. Fig. 226. -Silver fish [Lepisma saccharina), one of the Thysanura A primitive wingless insect common in pantries, feeding on sugary particles. It is towards half an inch long and covered with minute scales, which give it a slightly silvery appearance. A. A., Abdominal appendages; C.C, three caudal cerci. 4o6 PHYLUM ARTHROPODA (U >. to O X5 05 u 0) +-> o Oj o JS O CO > o CJ >^ o Sl t* rt ° xi ^i a5 h P. ^ 8^ CTi -l-> oJ O r-' , O J-l O 05 0; OJ rt J) O - +j > bf > - o 1 Pi <1^ +-" rt -i •? -^ ti •^ 05 rO - re ^ >^ ^ .2 -s 03 q:2 fli Ih ^ O ^ S b£-M O bJD o ^ o o (1) >+-, 0) Xi •I— I O 05 0) 0) > Pn.t; t/) ^ -4-> ... 05 > o; u > > u 03 42 o; o CD oi d ■- ■5 a; rd H ^ •+-'>. a; . — ■ +-> OJ rt rt d C/) ■ m in ft d i> d 05 rd 03 dj '-' > 0) tu t/; C« d *'*^ r/l r-* t/) ^-' d . 8 v> a; o -?x. 03 p- O d bx) 03 03 l-l 05 p. o cu a; ^ <^;d 03 u d O <" >> ti ^PQ •h 0^ t3 'i ^ § g W . ,d > 0) •5 o -d :" Q ^ (/, o >. o b£ IT. in > '• ^ '^ ^ Hips bJD O oi d +-> ;::; ^- "^ "3 X . ■a o ►-1 d d bf) -; b£' ti 03 1* -^ 1^ is rt a,* •^. 03 o 3 - ^ - d -^ k^ o CO 1) 'a; en 03 1^ br, O T3 oi 73 ' — ' r-i rt 71 & ci; a; en 2 g en tn . q=l sj _ d3 rt (V) o S ^ 03 1 <'' rt d > .2 ;-< rd I— I ;:3 -d H en d > d d 'S3 en S C 05 'w fl o :d OJ rd " -w c3 & -^ bij cti d /:: rtH S.2 -^^ >. 03 en en tj iS ^ bi; Q, d en 2'3 e^2 O Pi O en e d .'S \o 0) Ui O tD ;-i O Ul o u (L> V-l o M Ui (D u c d « biD -^ 2 o en I- d d 9 2 K J', O Xi u 73 o u T3 Ui o S-i 0) Ui O 00 Ui i-i O ^^ *> biD d -l-l •3 sects :orms Ifl 4:! ^ d . d ^ a 03 tn en ►—1 D ^ > be 03 ;=!^ w , tfl a '" u (U C +-> TZJ HH OS S.2 a; rl sf-;;! 0) C3 oi a; 3 J;^ 'd ^^ '^. w 3 -t-> ■« 0) V.;^ ft H m <+-! *■ •Td a; rt -^ rt o u a Ul > m bC V-i ft !-l o d bfi d fcjD d CO d 1— 1 03 (U tn o bC+i d d IH o •^ d -. 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Ametabo i.e. with no metam and Hemimeta with incomplete me d;5 S ft (—1 • "^ ^ •:•■; ■■>::::■( (•.:v^-,i'f..:-7 •*...' • • • • j§ m ■'-■■■-■■.•:( '•'■'■'-'■i ' •■ -'.'1 ¥ Fig. 234. — Follicle-mite Fig. 235. (greatly enlarged). -Itch-mite (Sarcoptes scabiei) (greatly enlarged). animals (Fig. 234). Gall-mites (Phytoptids), forming dimples and pouches on plants. {b) With tracheae. Harvest-mites {Trombidium), whose minute hexapod larvaj are troublesome parasites in summer on insects, many mammals, and man. The so-called " red spider " {Tetranychus telearius) spins webs, and lives socially under leaves. Water-mites, e.g. Hydrachna on water- beetles, and Atax on gills of fresh-water mussels. Beetle- mites {Gamasiis), often found on carrion beetles and on humble-bees. There is a common red mite on the shore- rocks, known as Molgus (Bdella) litioralis. 424 PHYLUM ARTHROPODA (c) With or without trachcce. In Acarapis woodt, parasitic in the thoracic tracheas of hive-bees suffering from Isle of Wight disease, the female breathes by trachea^, the male simply by the skin. Ticks (Ixodida?, etc.) are the largest Acarina. They show a movable " capitiilum " bearing serrated cutting chelicercT? and strong four-jointed pedipalps. They are responsible for spreading the germs of some diseases affecting man and beast, e.g. hum.an " tick-fever " on the Congo, spread by Ornithodoros mouhata ; a spirochaete disease in poultry, borne b}^ Argas reflexus and A. persicus ; Texas fever or " red water " in cattle, carried by Boophilus annulatus. The common sheep-tick in Fig. 236. — Tick {Ixodes riduvius, female), dorsal surface, showing the oval shield (S//.).— After Wheler. H., Hypopharynx ; P., palp L.I., L.IV., first and fourth leg. Fig. 237. — Tick {Ixodes riduvius, female), ventral surface. — After Wheler. R., Rostrum ; P., palp ; G., genital aperture; ST., stigma ; /I. , anus. Britain is Ixodes ricinus. It may be noted that mites have been found inside human tumours, and there are many facts suggesting that some of the small Acarines may share in spreading disease germs. Even Deviodex may play its part. Aberrant Orders Order Linguatulida or Pentastomida, e.g. Pentastomum tcenioides This strange animal is parasitic in the nasal and frontal cavities, etc., of the dog and wolf. It is worm-like in form, externally ringed. MOUTH-PARTS OF TICK 425 without anv oral appendages, but with two pairs of movable hooks near the mouth.' The muscles are striated. The alimentary canal is very Fig. 238.— Mouth-parts of a tick (one of the Ixodida?). — From a Specimen, seen from the veritral aspect. I and 2. The capitulum or " false head " from beneath. The transverse line between I and 2 does not seem to have any significance. 3. The fixing hypostome with recurved teeth, a median process from behind the mouth-opening. . 4. The cutting chelicera, with curious out-turned teeth at the tip. The chelicera is sometimes called the mandible. 5. The blade of the chelicera. 6. The pedipalp or palp. I.-IV. Its four joints. When the pedipalps are apposed, a groove on their inner aspect receives the chelicerae and hypostome. simple, without Malpighian tubes. A narrow circumoesophag. al nerve- ring, without a brain, is connected with a single ventral ganglion. 426 PHYLUM ARTHROPODA There are no sense organs nor tracheae, nor is there any heart. The sexes are separate ; the males smaller than the female. Embryos within egg-cases pass from the nostrils of the dog. If they happen to be swallowed by a rabbit or a hare, or it may be some other mammal, the embryos hatch in the gut and penetrate to liver or lung. There they encyst, moult, and undergo metamorphosis. The final larval form has two pairs of short legs, and has been compared to a larval mite. Liberated from its encystment, it moves about within its host, but will not become adult or sexual unless its host be eaten by dog or wolf. There are a few other species occurring in Reptiles, Apes, and even man, but their history is not adequately known, and the systematic position is very uncertain. There is very little reason for ranking them along with Arachnoids. Order Tardigrada. Water-bears or Sloth-animalcules, e.g. Macrobiotus Microscopic animals, sometimes foimd about the damp moss of swamps or even in the roof-gutters of houses. Some occur in fresh water, others in the sea. The unsegmented body is somewhat worm- like, with four pairs of unjointed clawed limbs like little stumps, with mouth-parts resembling those of some mites, and adapted for piercing and sucking. The muscles are unstriped. There is no abdomen. There is a food canal, a brain, and a ventral chain of four ganglia, sometimes even a pair of simple eyes, but no respiratory or vascular organs. The sexes are separate ; the males rarer and smaller. The terrestrial Tardigrada, even as adults, have great powers of successfully resisting desiccation, but sometimes only the eggs do so, developing rapidly when favourable conditions return. There is very little reason for ranking them along with Arachnoids. Perhaps, as the seta-like " claws " and the cirri of some types suggest, they are nearer to Annelids. Class Pal^ostraca The three following orders, Xiphosura, Eurypterina, and Trilobita, may be united under this title. They live or lived in water, and have or had gills in association with the limbs. The well-developed antennae of Trilobites, to- gether with the markedly biramose character of some of their limbs, suggest an affinity with Crustacea, but, on the other hand, the affinities of the Xiphosura seem to be distinctly Arachnoid. Order i. Xiphosura There is one living genus, the King-crab or Horseshoe- crab {Limulus). The King-crab lives at slight depths off the muddy or KING-CRAB 427 sandy shores of the sheUered bays and estuaries of North America, from INIaine to Florida, in the West Indies, and also on the Molucca Islands, etc., in the Far East. The body consists of a vaulted cephalothorax shaped like a horseshoe, and an almost hexagonal abdomen ending in a long spine. Burrowing in the sand, Limulus arches its body at the joint between cephalothorax and abdomen, and pushes forward with legs and spine. It may also walk about under water, and even rise a little from the bottom. It is a hardy animal, able to survive exposure on the shore, or even some freshening of the water. Its food consists chiefly of worms. The King-crab is interesting in its structure and habits and also because it is the only living representative of an old race. The hard, horseshoe ■ shaped, chitinous cephalothoracic shield is vaulted, but the internal cavity is much smaller than one would at first sight suppose ; the well-defined abdomen shows some hint of being divisible into meso- and meta-soma ; the long sharp spine is (like the scorpion's sting) a post-anal telson. On the concave under-surface of the cephalothorax there are six (or seven) pairs of limbs, as in spiders and scorpions — (i) A little pair of three-jointed chelicerae in front of and bent towards the mouth. (2) A pair of pedipalps lateral to the mouth. (3-6) Four pairs of walking legs, the bases of which surround the moiith, and help in mastication. Behind these, still on the cephalothorax, there is a pair of small appendages called chilaria. Then follows on the abdomen a double " operculum " with the genital apertures on its posterior surface. Under the operculum lie five pairs of flat plates bearing remark- able respiratory organs ("gill-books"). These appendages show hints Fig. 239. — Limulus or King-crab. clr:, Chelicerse ; op., operculum ; a., anus. 428 PHYLUM ARTHROPODA Fic. 240. — Limulus or King-crab, dorsal surface. — From a Specimen. CPH., Anterior margin of vaulted cephalothorax or pro-soma. M.E., Median eyes ; L.E., lateral eyes. AB., Toothed lateral margin of the abdomen, otherwise called meso- soma and mcta-soma. 7"., The post-anal spine or telson. PAL^EOSTRACA 429 of the exopodite and endopodite structure characteristic of Crustaceans. Each " gill-book " looks like a much-plaited gill, or like a book with over a hundred hollow leaves. The leaf-like folds are externally washed by the water, and within them the blood flows. The leaves of the gill-books are often compared to the leaves of the insunk lung- books of scorpions. Spawning occurs in the spring and summer months. The ova and spermatozoa are deposited in hollows near high-water mark. Some of the early stages of development present considerable resem- blance to corresponding stages in the scorpion. In the larvae, both cephalothorax and abdomen show signs of segmentation, but this disappears. The spine is represented only by a very short plate, and the larva presents a striking superficial resemblance to a Trilobite. It seems likely that Limulus is linked to the extinct Eurypterids by some fossil forms known as Hemiaspidae, e.g. Hemiaspis, Belinurus. Order 2. Eurypterina ( = Mero- stomata or Gigantostraca), e.g. Eurypterus Large extinct forms found from Cambrian to Carboniferous strata. The body is divided into head, thorax, and abdomen. The head is small and unsegmented. The thorax is composed of six distinct segments, the abdomen of six with a terminal telson. On the head are borne six pairs of appendages of varying shape, two lateral compound eves, and two median ocelh. On the ventral surface of the thorax there are five pairs of gills covered bv flat plates, of which the most anterior pair are verv large, and form the so-called operculum (cf. Limulus). The surface of the body was covered with scales. Some of the Eurvpterids reached a length of 6 ft. The oldest Merostomes are referred by Walcott to a sub-order Limulava somewhat divergent from other Eurypterids. This order is sometimes placed near the Crustacea, but the general opinion is that they are linked through Limulus to Arachnoids. Fig. 241. — Young Limulus. — After Walcott. Order 3. Trilobita. Trilobites, e.g. Calymene, Phacops, Asaphus Extinct forms chiefly found in Cambrian and Ordovician strata, but extending up to the Carboniferous. The body as found is divisible into three parts- -the unsegmented head shield, often prolonged back- 430 PHYLUM ARTHROPODA wards at the anij;les ; the flexible thorax of a varying number of segments ; the unsegmented abdomen or pygidium. A median longitudinal ridge, or rachis, divides the body into three longitudinal portions. Traces of limbs are only rarely preserved. In the head region there are four pairs, apparently simple. Segmented antennas have been found in this region. The thorax and abdomen are furnished with bira- mose appendages, with long-jointed endopodite, shorter exopodite. and a gill (or epipodite ?) of varying shape. In the abdominal region the gills were perhaps rudimentar}'. Trilobites are often found rolled up in a way that reminds one of some wood-lice. So abundant arc they in some rocks that even their development has been studied with some success. The limbs seem to be more like those of Crustaceans than those of Arachnoids, and the antennae, observed (1759), and securely accentuates the resemblance. The affinities with Limulus, according to the views of other authorities, justify the association of Trilobites and Arachnoids. A compromise Fig. 242.- aliies- --Trilobite {Conoceph- — After Barrande. li.s., Head shield ; pL, pleura of thoracic region ; py., pygidium. occurrence of by Linnaeus corroborated, Fig. 243. — Vertical cross-section of a Trilobite (Calymene). —After Walcott. i., Intestine ; s., shield ; L., endopodite ; e., exopodite ; b., epipodial parts. may be perhaps effected by regarding the Trilobites as an offshoot from a stock ancestral to both Arachnoids and Crustaceans. SEA-SPIDERg 431 IncertcE Sedis Class Pycnogonida, Pantopoda, or Podosomata Marine Arthropods, sometimes called sea-spiders. They may be ranked between Crustaceans and Arachnoids. Many climb about Fig. 244. — Under -surface of a Trilobite. — After Beecher. I, Antennae ; 2, mouth-appendages ; 3, the under-surface of the head shield ; 4, the biramose thoracic hnibs; 5, the biramose abdominal limbs ; M., position of the mouth. seaweeds and hydroids near the shore, but some live at great depths. The body consists of an anterior proboscis, cephalothoracic region 432 PHYLUM ARTHROPODA with three fused and three free segments, and an unsegmented rudi- mentary abdomen. Four somewhat primitive eyes on an anterior hillock are nearer to the eyes of Arachnoids than to those of any other class. There are typically seven pairs of append- ages. The first are short and chelate, but may be absent in the adult. The next two are small and slender, and are often absent in the adult female ; the second pair may also be absent in the male, but the third in the males of all genera carries the eggs. The last four pairs of appendages are always present, and form the walking legs. Into them, and into the cheliceraj when these are present, out- growths of the mid-gut extend. The sexes are separate. The larvse are at first unsegmented, with three pairs of appendages. Fig. 245. — Sea-spider (Pycnogonum litturale), from the dorsal surface. Fig. 246. — Male of Nymphon. — After Sars. PR., Proboscis ; CH., chelophores ; P., pedipalps ; £., eggs carried on ovigerous legs ; A., rudimentary abdomen. Examples. — Pycnogonum, Nymphon, Ammothea. In Pentanymphon and Decolopoda there is an extra pair of long walking legs. CHAPTER XVI PHYLUM MOLLUSCA Classes : — i. Gasteropoda, e.g. Snails. 2. Solenogastres — A small class of doubtful worm-like forms, e.g. Neomenia. 3. Scapho- PODA — A small class, e.g. Dentalium. 4. Lamellibranchiata — Bivalves. 5. Cephalopoda — Cuttle-fishes. The series of Molluscs is in many ways contrasted with that of Arthropods ; thus the body of the Mollusc is un- segmented, and there are no appendages. The general habit of life is also very different, for, although there are active Molluscs and sluggish Arthropods, it is true as an average statement that Molluscs are sluggish and Arthro- pods are active. In the frequent presence of a trochosphere larva, in the nerve-ring around the gullet, and in some other features. Molluscs resemble Annehds, but it is probable that they took their origin from a still lower level. General Characters Molluscs are unsegmented and without appendages. The symmetry is fundamentally bilateral, hut this is lost in most Gasteropods.' The ''foot''— a muscular protrusion of the ventral surface — is very characteristic ; it usually serves for locomotion , but is much modified according to habit. Typically , a dorsal or lateral fold of the body wall forms a mantle, or pallium (Fig. 247, c), which often secretes a single or bilobed shell covering the viscera, and encloses a space — the mantle cavity^within which lie the gills. But both mantle and shell may be absent. There are three chief pairs of ganglia — cere- brals, pedals, and pleurals — with connecting circumoesophageal commissures^ and there is also a visceral nervous system con- 28 434 PHYLUM MOLLUSCA sisting typically of (a) a loop connecting the two pleurals and provided with tvjo visceral ganglia, and (b) a stomato-gastric loop connecting the cerebrals below the gullet and provided with two buccal ganglia (Fig. 247). Except in Lamelli- branchs, in which the head region is degenerate, there is in the mouth a chitinous ribbon or radula, usually bearing numerous small teeth, and moved by special muscles, the whole structure being known as the odontophore. There is much unstriped muscle, but the more rapidly contracting muscles have cross- striped fibres, or fibres with unstriped fibrils twisted in a Fig. 247. — Ideal mollusc. — After Ray Lankester. m., Mouth ; g.c, cerebral ganglia ; c, edges of mantle skirt ; z.g., duct of right lobe of digestive gland ; s., pericardial cavity ; /., edges of shell-sac ; i;., ventricle of heart ; j<., nephridium ; an., anus ; n., posterior part of the foot ; /., opening of osphradiiim ; k., genital aperture ; g.ab., abdominal ganglion on visceral loop ; g.v., visceral ganglion ; z.l., left lobe of digestive gland ; p. foot ; g.pe., pedal ganglion ; g.pL, pleural ganglion. Spiral. A portion of the true body cavity or coelom usually persists as the pericardium at least (Fig. 247, s.), and communicates with the exterior through the nephridium or nephridia. The rest of the cavity of the body is hcemocoelic. The vascular system is almost always well developed, but part of the circulation is in most cases lacunar ; the heart typically consists of a ventricle and two auricles. Respiratory organs are most typically represented by gills or ctenidia, consisting of an axis attached to the body and bearing lamella, but the gills may have simpler forms, or may be absent, and in the terrestrial snails the mantle cavity is adapted for aerial respiration. At the base of the gills there is generally MOLLUSCAN LARV^ 435 an olfactory organ or osphradiiim. The sexes are separate or united. There are two common larval stages — the Fig. 248. — Stages in molluscan development. I. Larva of Heteropod (after Gegenbaur) ; sh., shell covering visceral hump ; v., vehim ; /., foot. II. Larva of Atlanta (after Gegenbaur) ; v., velum ; sh., shell ; /., foot ; op., operculum. Trochosphere, which resembles the same stage in some Annelids, and the more characteristic Veliger (Fig. 248) ; but the development is often direct. 436 PHYLUM MOLLUSCA First Type of Mollusca. The Snail (Helix), one of the terrestrial (pulmonate) Gasteropods Habits. — The common garden snail (H. aspersa), or the larger edible snail [H. pomatia), which is rare in England but abundant on the Continent, serves as a convenient type of this large genus of land-snails. They are thoroughly terrestrial animals, breathing air directly through a pulmon- ary chamber, and drowning (slowly) when immersed in water. Their food consists of leaves and other parts of plants, but they sometimes indulge in strange vagaries of appetite. They are hermaphrodite, but there is always Fig. 249. — Roman snail {Helix pomatia). Note shell covering visceral hump ; p.ap., pulmonary aperture (hicluding anus and opening of ureter) ; /., the foot ; g.ap., genital aperture ; m., mouth ; e., eye on long horn ; s.Ji., one of short horns. cross-fertilisation. The breeding time is spring, and the eggs are laid in the ground. In winter snails bury them- selves, usually in companies, cement the mouths of their shells with hardened mucus and a little lime, and fall into a state of " latent Ufe," in which the heart beats feebly. They have been known to remain dormant for years. General appearance. — A snail actively creeping shows a well-developed head, with two pairs of retractile horns or tentacles, of which the longer and posterior bear eyes. The foot, by the muscular contraction of which the animal creeps, is very large ; it leaves behind it a trail of mucus. The viscera protrude, as if ruptured, in a dorsal hump, which is spirally coiled and protected by the spiral shell. SHELL OF SNAIL 437 On slight provocation the animal retracts itself within its shell, a process which drives air from the mantle cavity, and thus helps indirectly in respiration. Around the mouth of the shell is a very thick mantle margin or collar, by which the continued growth of the shell is secured. On the right side of the expanded animal, close to the anterior edge of the shell, there is a large aperture through which air passes into and out of the mantle cavity. Within the same aperture is the terminal opening of the ureter. The food canal ends slightly below and to the right of the pulmonary aperture. All the three openings are close together. The Fig. 250. — Vertical section of the shell of a species of Helix. M., Mouth of shell ; A., apex ; C, columella. anterior termination of ureter and food canal is one of the results of the twisting of the visceral mass forwards to the right. But still farther forward, at the end of a slight groove which runs along the right side of the neck, indeed quite close to the mouth, is the genitaf aperture. Lastly, an opening just beneath the mouth leads into the large mucus gland of the foot. Shell. The right-handed spiral shell is a cuticular product made and periodically enlarged by the collar. Chemically it consists of carbonate of hme and an organic basis (conchin). The outermost layer is coloured, without lime, and easily rubbed off ; the median layer is thickest, and looks like porcelain ; the innermost layer is pearly. The twisted cavity of the shell is continuous, and the viscera extend to the uppermost andj.'oldest part. 438 PHYLUM MOLLUSCA As the shell is made, the inner walls of the coils form a central pillar (columella), as on a staircase, to which the animal is bound by a strong (columellar) muscle. Many Gasteropods bear on the foot a lid or operculum, of conchin or of lime, which closes the mouth of the shell. In Helix there is none ; the " epiphragm " with which the shell is sealed in winter consists of hardened mucus, plus phosphate and a smaller quantit}^ of carbonate of lime. It is formed very quickly from the collar region when cold weather sets in, has no organic connection with the animal, such as binds the operculum to the foot of the whelk, and is loosened off in the mildness of spring. Sinistral shells, with left-handed spiral, occasionally occur as variations. The shell, held summit upwards and mouth towards the observer, has the mouth to the left. The internal organs are inverted, and at the start there is a reversal of the cleavage planes of the egg. Appearance after the shell is removed. — If the shell is removed carefully, so that nothing is broken except the columellar muscle, many structures can be seen without any dissection. The skin of the head and foot should be contrasted — {a) with the thick collar of the mantle ; (b) with the mantle itself, which forms the loose roof of the pulmonary chamber ; {c) with the exceedingly delicate, much-stretched, and always protected skin of the visceral hump. The mantle is a downgrowth of the skin of this dorsal region. It is peculiar in the snail, in that its margin (the collar) is fused to the body wall. The result is to form a respiratory cavity, which is as much outside the body as is the gill-chamber of the crayfish. It is important to realise that the great rupture-like hump of viscera on the dorsal surface has been coiled spirally, and that there is the yet deeper torsion forward to the right. A great part of the hump consists of the greenish brown digestive gland, in which the bluish intestine coils ; behind the mantle chamber, on the right, lies the triangular and greyish kidney ; the whitish reproductive organ lies in the second last and third last coil of the spiral. Skin. — This varies greatly in thickness. It consists of a single-layered epidermis and a more complex dermis, including connective tissue and muscle fibres. There are numerous cells from which mucus, pigment, and lime are secreted ; those forming pigment and lime are especially abundant on the collar, where they contribute to the growth of the shell. Muscular system. -7- Among the important muscles are — NERVOUS SYSTEM OF SNAIL 439 (a) those of the foot ; (h) those which retract the animal into its shell, and are in part attached to the columella ; (c) those which work the radula in the mouth ; (d) the retractors of the horns : and (e) the retractor of the penis. The muscle fibres usually appear unstriated. There is much connective tissue, some of the cells of which contain glycogen, pigment, and lime. Nervous system. — This is concentrated in a ring around the gullet. Careful examination shows that this ring consists dorsally of a pair of cerebral ganglia, con- nected ventrally with a pair of pedals and a pair of pleuro- viscerals, which, according to some authorities, have a median abdominal ganglion lying between them. The cerebrals give off nerves to the head, e.g. to the mouth, tentacles, and otocysts, and also two nerves which run to small buccal ganglia, lying beneath the junction of gullet and buccal mass. The pedals give off nerves to the foot ; the pleuro-viscerals to the mantle and posterior organs. Sense organs. — An eye, innervated from the brain, is situated on one side of the tip of each of the two long horns. It is a cup invaginated from the epidermis, lined posteriorly by a single layer of pigmented and non-pigmented retinal cells, filled with a clear vitreous body perhaps equivalent to a lens, closed in front by a transparent " cornea," and strengthened all round by a firm " sclerotic." How much a snail sees we do not know, but it detects quick movements. Though the eye is by no means very simple, the snail soon makes another if the original be lost, and this process of regeneration has been known to occur twenty times in succession. The otocysts appear as two small white spots on the pedal ganglia. Each is a sac of connective tissue, lined by epithelium which is said to be ciliated in one region, containing a fluid and a variable number of oval otoliths of lime, and innervated by a delicate nerve from the cere- bral ganglia. Though no osphradium or smelling-patch, comparable to that which occurs at the base of the gills in most Molluscs, has been discovered in Helix, the snail is repelled or attracted by odours ; it shrinks from tur- pentine, it smells strawberries from afar. This sense of smell seems to be located in the horns, for a dishorned snail has none. The tips of both pairs of horns bear sensory cells connected with ganglionic tissue and nerve-fibres within. Other sensory cells, probably of use in tasting, lie on the lips ; and there are many others, which may be called tactile, on the sides of the foot, and on various parts of the body. In short, the snail is diffusely sensitive. 440 PHYLUM MOLLUSCA Alimentary system.— In cutting a piece of leaf, the snail uses two instruments — the crescentic jaw-plate on the roof of the mouth, and the toothed ribbon or radula on the floor. This radula is like a flexible file — a short and broad strip of membrane, bearing several longitudinal rows of minute chitinoid teeth. It rests on a cartilaginous pad on the floor of the mouth cavity, and is moved (backwards and forwards, and up and down) in a curve by protractor and retractor muscles. The whole apparatus, including teeth, mem- brane, and pad, is called the odontophore. The radula wears away anteriorly, but is added to posteriorly within a radula sac which projects from the floor of the buccal cavity. Its action on leaves may be compared very roughly to that of a file, but its movements within the mouth also produce a kind of suction which draws food particles inwards. In this suction the muscular lips and the cilia in the mouth cavity assist. The ducts of two large salivary glands open on the dorsal surface of the buccal cavity, and there are numerous distinct glandular cells close to the entrance of the two ducts. The salivary glands are large lobed structures, and extend far backward on the crop. They consist of hundreds of glandular cells or unicellular glands, which secrete a clear fluid. This travels up the ducts, and is forced, in part at least, by muscular compression, into the buccal cavity. While some say that this fluid converts starch into sugar (after the usual fashion of saliva), other authorities deny that it has any eflFect upon the food. Similar glands are found in all Gasteropods, while they are entirely absent in Lamellibranchs. In some boring Gasteropods the secre- tion contains 2-4 per cent, of free sulphuric acid. The gullet extends backward from the buccal cavity, and expands into a storing-crop ; this is followed by a small stomach surrounded by the digestive gland ; thence the intestine extends, and, after coiling in the visceral hump, passes forward to end on the right side anteriorly beside the respiratorv aperture. The digestive tract is muscular, and in part ciliated internally. A large part of the visceral spiral is occupied by the so- called " liver." This gland has two lobes, each of which opens by a duct into the stomach. The left lobe is again VASCULAR SYSTEM OF SNAIL 441 imperfectly divided into three. Besides producing juices which digest ail kinds of food, the gland makes glycogen, stores phosphate of lime, and contains a greenish pigment. It is thus more than a " liver," more even than a " hepato- pancreas," it is a complex digestive gland, producing several digestive ferments. The phosphate of lime may possibly be used to form the autumnal epiphragm. Vascular system. — The blood contains some colourless amoeboid cells, and a respiratory pigment called haemo- cyanin, which gives the oxidised blood a blue tint, and is very common among Molluscs. The heart, with a ventricle and a single auricle, lies in a pericardial chamber on the dorsal surface, to the left side, behind the mantle cavity. The average number of pulsa- tions in Gasteropods is about one hundred per minute, but in the hibernating snail the beating is scarcely perceptible. From the ventricle pure blood flows by cephalic and visceral arteries to the head, foot, and body, passes into fine ramifications of these arteries, and thence into spaces among the tissues. From these the blood is collected in larger venous spaces, and eventually in a pulmonary sinus around the mantle cavity, on the roof of which there is a network of vessels. There the blood is purified. Most of it returns directly to the auricle by a large pulmonary vein, but some passes first through the kidney. Respiratory system. — Most Gasteropods, e.g. the dog- whelk {Purpura), the buckie (Buccinum), the periwinkle {Ltttorma), breathe by gills covered by the mantle. The snail, being entirely terrestrial, has a pulmonary or lung cavity, formed by the mantle fold. On the roof of this cavity the blood vessels are spread out. Air passes into and out of the pulmonary chamber by the respiratory aperture. When the animal is retracted within its shell, the freshening of the air in the pulmonary chamber takes place by slow diffusion, but when the snail extends itself at full length, the chamber is rapidly filled with air, and it is even more rapidly emptied when the body is withdrawn into the shell. Excretory system. — ^There is a single triangular greyish kidney behind the pulmonary chamber, between the heart and the rectum. It is a sac with plaited walls, and excretes nitrogenous waste products, which pass out by a long ureter 442 PHYLUM MOLLUSCA running along the right side of the pulmonary chamber, and opening close beside the anus. There are two sources of blood supply to the kidney — {a) from the pulmonary chamber, and {b) from the heart by a renal artery. As in Fig. 251. — Dissection of snail. T., Short horn ; TT., long horn with eye ; N., cerebral ganglia ; S.G. salivary glands on the crop ; F., foot ; M., coluniellar muscle ; V.C., visceral coil ; O.T., ovotestis ; V., ventricle of heart ; R., rectum ; U., ureter ; B.V., blood vessels returning to the auricle from the mantle ; A., pulmonary aperture ; MA., edge of the mantle. most Other Molluscs, the kidney communicates by a small aperture with that part of the coelom which forms the pericardial sac. Thus, as in earthworm, lobworm, etc., the coelom has a nephridial connection with the exterior. Reproductive system. — The snail is hermaphrodite, and its reproductive organs exhibit much division of labour. GONADS OF SNAIL 443 {a) The essential reproductive organ (the ovotestts) is a whitish body near the apex of the visceral spire. It consists of numerous cylindrical foUicles, in each of which both ova and spermatozoa are formed, but not at the same time. {b) A much-convoluted hermaphrodite duct of a white O.T Fig. 252. — Reproductive organs of Helix pomatia. — After Meisenheimer. OT. Ovotestis ; H.D., hermaphrodite duct ; /I .G., albumen gland ; F.D.. female side' of common duct; M.D., male side of common duct ; 0., oviduct ; R.S., receptaculum seminis ; M.G., mucus glands ; D.S., dart-sac'; T.D., vas deferens ; FL., flagellum ; P., penis ; M., retractor muscle o'f penis'; AP., genital aperture. colour conducts the sex cells from the ovotestis, and leads to- the base of a large yellowish albumen gland. (c) This tongue-shaped albumen gland varies in size with the age and sexual state of the snail. It forms gelatinous protein material, which envelops and probably nourishes the ova. (d) The ova and spermatozoa pass from the herma- phrodite duct towards the head along a common duct, but 444 PHYLUM MOLLUSCA not at the same time. Moreover, their paths are different, for the portion of the duct down which the ova travel is much plaited, while the path which the spermatozoa follow is a less prominent groove, incompletely separated from the other. Both paths are glandular, and the glands on the male side are often called prostatic. (e) At the base of this comm.on duct, a distinct vas deferens diverges to the left and leads into a muscular penis, which can be protruded at the single genital aperture and retracted by a special muscle. Before the vas deferens enters the penis, a long process or ftagelhim is given off. It is like the lash of a whip, and is as long as the common duct. Its secretion is used in forming a sperm-packet or spermatophore of a large number of sper- matozoa, which are com- pacted together at the time of sexual union partly in the flagellum, partly in the penis. The spermatophore is trans- ferred by the penis into ¥iG. 2ss.Sn3.i\ {Helix pomatia) Xa^ymg the genital aperture of its eggs. — After Meisenheimer. another snail. ( f) Continued from the oviducal side of the common duct, there is a separate ciliated oviduct. This has a short course, and ends in the common genital aperture. Before it reaches this, however, the oviduct is associated with two structures. The first of these is a long process, as long as the common duct beside which it runs, in appearance suggesting the flagellum, but expanding at its free end into a globular sac — the receptaculum seminis or spermatheca. In Helix aspersa a long slender diverticulum is given off from the duct of the receptaculum. This is also occasion- w^ ■/f ■ X^^r^^'-l^. GONADS OF THE SNAIL 445 ^- • C. - - ally seen in Helix pomatia. A spermatophore from another snail passes into the receptaculum, and is there dissolved after some days, liberating hundreds of spermatozoa. By these spermatozoa the ova of the snail are fertilised. It seems likely that the place of fertilisation is in a small diverticulum at the upper end of the oviducal side of the common duct, whither the spermatozoa are said to find their way. The second structure associated with the female duct is a conspicuous mucus gland, formed of two sets of finger-like processes. The secretion is very abundant during copulation, and as it contains not a little lime, it is possible that it may form the calcareous shells of the eggs. It seems to serve as a lubricant which facilitates the expulsion of a calcareous dart and the copulation. {g) Finally, between the entrance of oviduct and penis into the ter- minal aperture there lies a firm cylindrical struc- ture, larger than the penis and with muscular walls. It is the Cupid's Dart Sac, and contains a pointed calcareous arrow {spicu- lutn amoris), which is jerked out previous to copulation. The dart is sometimes found adhering to the foot of a snail, and after copulation the sack is empty, soon, however, to be refilled. When two snails pair, the genital apertures are dilated, the protruded penis of one is inserted into the aperture of the other, and the spermatophore of each snail is transferred to the receptaculum of the other. The large eggs are laid in the earth in June and July. Each is surrounded by gelatinous material acquired in the oviduct and by an elastic but calcareous shell. Segmentation is total but slightly unequal. As the snail is a terrestrial Gasteropod, there is no trochosphere Fig Ec big c.p. 254. — Diagram of larva of Palu- dina. — After Erlanger. Ectoderm; En., endoderm ; V., velum, with cilia ; g., gut-cavity ; S.c, segmenta- tion cavity ; c.p., coelom pocket from gut ; bl.g., blastopore groove closed, except at bl., which becomes the anus. The origin of the mesodenn from a gut-pocket is clearly seen in Paludina. 446 PHYLUM MOLLUSCA larva, nor more than a slight hint of the characteristic Molluscan velum. A miniature adult is hatched in about three weeks. The study of development may be more profitably followed in the pond-snail Limnceus, where gastrula, trochosphere, and veliger can be readily seen. Second Type of Mollusca. The Fresh-w^ater Mussel [Anodofita cygnea), one of the Lamellibranchiata Habit. — The fresh-water mussel lives in rivers and ponds. It lies wdth its head end buried in the mud, or moves slowly along by means of its ploughshare-like foot. Its food consists of minute plants and animals, which are wafted in at the posterior end by the currents produced by the ciliated gills. What is noted here in regard to Anodonta will also apply, for the most part, to Unio and other fresh- water mussels. External appearance. — The bivalve is 4 to 6 in. long ; its valves are equal and united in a dorsal hinge by an elastic ligament, an uncalcified part of the shell ; on the ventral surface when the valves gape the foot protrudes ; the anterior end is rounded, the posterior end is more pointed, and it is there that the water currents flow in (ventrally) and out (dorsally). In bivalves the ligament is generally posterior to the dorsal knob or umbo — the oldest part of the shell — and the umbo generally points towards the anterior end. The greenish brown soft (" horny ") layer of the shell is often worn away near the umbo on each side, and then displays the median layer of lime. This is called prismatic, since the lime salts are deposited in prisms, transversely varicose or striated, like those w-hich form the enamel of our teeth. Internally there is a pearly layer. Lines of growth on the shell mark the position of the margin in former years, the newest part being obviously at the edge. The shell is a cuticular structure, i.e. it is made by the epidermis of the mantle. It consists, as in the snail, of calcium carbonate plus conchiolin or conchin. Thus the composition of a Pinna shell is : — Lime salts, 89-2 ; organic matrix, 1-3 ; water, 9-5. Internal appearance. — When the right half of the shell a.r Fig. 255. — The fresh-water mussel {XJnio). The uppermost figure represents the bivalve in motion in the mud with protruded foot (F.) ; note inhalant and exhalant aper- tures. The middle figure shows the inside of the shell (left valve). The lower figure shows the outside (right valve). «., The umbo; L., the ligament ; c./., lateral teeth ; a.a., anterior adductor mark ; a.r., mark of protractor of the foot ; p.h, pallial line ; p.a., posterior adductor mark ; p.r., mark of posterior retractor of the foot; l.g., a line of growth; A., anterior (the blunter end) ; P., posterior ; V ., ventral. 44^ PHYLUM MOLLUSCA is folded back, the anterior and posterior closing muscles having been carefully cut close to the gently raised valve, the mantle folds are seen lining the shell, and forming posteriorly the ventral inhalant and dorsal exhalant lips. The ventral lips have papillary processes. Internal to the mantle there are two gill-plates on each side ; projecting from between these is the foot, muscular ventrally, softer dorsally ; the median dorsal pericardium is just beneath the ligament ; the ventricle shines through its walls, and the dark-coloured kidneys are seen through its floor. Below the anterior adductor muscle is the large mouth, bordered beneath by two lip processes (labial palps) on each side. These resemble the gills in appearance, and are probably modified portions of the gills. The anus is above the posterior closing muscle. The whole space between the two mantle flaps is called the mantle cavity, and it is divided by a shght partition at the bases of the gills into a large ventral infra-branchial chamber and a small dorsal supra-branchial chamber which ends at the exhalant orifice. On the surface of the valves of the shell a few small pearls may be seen ; they are formed by the enclosure of some minute grains of sand in the prismatic layer. There are two teeth in front of the umbo in Unio, but not in Anodonta. The following muscles are inserted on the shell, and leave impressions : — {a) The anterior adductor. [b) The posterior adductor. (o) The anterior retractor of the foot continues with (a). {d) The protractor of the foot a little below (a). {e) The posterior retractor of the foot continues with {b). As the shell grows, the insertion of the muscles and the attachment of the mantle change, and the traces of this shifting are visible. Skin. — There is much ciliated epithelium about Ano- donta, especially on the internal surface of the mantle, on the gills, and on the labial palps ; and little pieces cut from an animal incompletely dead {e.g. from the oyster swallowed half-alive) have by means of their cilia a slight power of motion. The skin of the foot is not ciliated but glandular ; on the mantle edge sensitive and glandular cells are abund- ant, but usuallv in inverse ratio to one another. TONUS OF MUSCLE 449 Muscular system. — The shell is closed and kept closed by the action of the two adductor muscles. When these are relaxed under nervous control, the elasticity of the hinge ligament opens the valves. The foot is a muscular protru- sion of the ventral surface, under the control of three muscles — a retractor and a protractor anteriorly, and a posterior retractor. Its upper portion contains some coils of gut and the reproductive organs ; its lower region is very muscular. The protrusion or extension of this locomotor organ is mainly due to an inflow of blood, which is pre- vented from returning by the contraction of a sphincter muscle round the veins. In moving, the animal literally ploughs its way along the bottom of the pond or river pool, and leaves a furrow in its track. The muscle fibres, as in the snail, are mainly of the slowly contracting non-striped sort, but those of the adductor and of the heart show oblique cross-striping. In that part of the adductor muscle of Pecten (and some other bivalves) that effects the rapid closing of the valves, and hence the swimming, the muscle- fibres are transversely cross-striped, and the same is true of those found in the margin of the mobile mantle. There is here therefore a good instance of the connection between striation and rapidity of contraction and relaxation. Tonus. — The adductor muscles of bivalves furnish a good example of a remarkable property of smooth muscle, that of maintaining a state of contraction with little or no expenditure of energy. If a piece of wood is inserted between the open valves, the adductors contract, and the valves close as far as they can. If the wood is twisted out, the valves remain as they were, without closing : it is impossible to pull the valves apart without tearing the muscle, but they can easily be pressed closer together, when they maintain their new position as firmly. The mechanism recalls that of a catch or ratchet. Similar behaviour is shown by smooth muscle in other Inverte- brates (sea-anemones, for instance, maintain either the contracted or the expanded state without expenditure of energy and without fatigue), and in the hollow viscera of Vertebrates. Striated muscle may behave in an analogous way, but only under the constant controlling influence of the nervous system. 29 450 PHYLUM MOLLUSCA Nervous system. — There are three pairs of nerve- centres : — (a) Cerebro-pleiiral ganglia, lying above the mouth on each side on the tendon of the anterior retractor of the foot, connected to one another by a commissure, connected to the two other pairs of gangha {b) and (c), by long paired connect- ives, and giving off some nerves to mantle, palps, etc. {b) Pedal ganglia, lying close together about the middle of the foot, united by connectives to {a), giving off nerves to the foot, and having beside them two small ear-sacs, each with a calcareous otolith, and with a nerve said to be derived from the cerebral ganglion. {c) Visceral ganglia (also called parieto-splanchnic or osphradial), lying below the posterior adductor, connected to {a) by two long connectives, and giving off nerves to mantle, muscles, etc., and to a patch of " smelling cells " {osphradium) at the bases of the gills. Sense organs. — Unlike not a few bivalves, which have hundreds of " eyes " on the mantle margin, Anodonta has no trace of any. The ear-sac, originally derived from a skin-pit, is sunk deeply within the foot, and is of doubtful use. The " smelling patch " or " osphradium " at the base of the gills has perhaps water-testing qualities. There are also " tactile " cells about the mantle, labial palps, etc. Alimentary system. — The mouth lies between the anterior adductor and the foot, and beside it he the cihated, vascular, and sensitive labial palps, two on each side, which waft food into the mouth. The ciliary currents are very complex, and effect a sifting of the large and small particles. The large mouth opens immediately into the gullet, for the pharynx of other Molluscs, with all its associated struc- tures, is absent in Lamellibranchs. The short wide gullet leads into a large stomach surrounded by a paired digestive gland. Part of the food digested by these juices in the stomach is compacted in autumn into a " crystalline style " — a mass of reserve food-stufFs, and similar but less solid material is found in the intestine. On this supply I^RESH-WATER MUSSEL 451 the mussel tides over the winter. In other bivalves other functions are ascribed to the crystalline style, which is thought to aid in digestion, both mechanically, for by rotating it stirs the food, and chemically, for it contains a starch-splitting enzyme. Wandering amcEboid cells play Fig. 256. — Structure of Anodonta. — After Rankin. a.a., Anterior adductor ; c.p.g., cerebro-pleural ganglia ; St., stomach ; v., ventricle, with an auricle opening into it ; k., kidney, above which is the posterior retractor of the foot ; r., rectum ending above posterior adductor ; v.g., visceral ganglia with connectives (in black) from cerebro-pleurals ; g., gut coiling in foot ; p.g., pedal ganglia in foot, where also are seen branches of the anterior aorta and the reproductive organs ; l.p., labial palps behind mouth.-- At the posterior end the exhalant (upper) and inhalant (lower) apertures are seen. a large part in digestion and absorption. The intestine, which has in part a folded wall like that of the earthworm, coils about in the foot, ascends to the pericardium, passes through the ventricle of the heart, and ends above the posterior adductor at the exhalant orifice. Vascular system. — The heart lies in the middle line on the dorsal surface, within a portion of the body cavity called 452 PHYLUM MOLLUSCA the pericardium, and consists of a muscular ventricle which has grown round the gut and drives blood to the body, and of two transparent auricles — one on each side of the ventricle — which receive blood returning from the gills and mantle. In bivalves the heart-beats average about twenty per minute, far fewer than in Gasteropods. The colour- less blood passes from the ventricle by an anterior and a posterior artery ; flows into ill-defined channels ; is collected in a " vena cava " beneath the floor of the peri- cardium ; passes thence through the kidneys, where it loses nitrogenous waste, to the gills, where it loses car- bonic acid and gains oxygen ; and returns finally by the auricles to the ventricle. The blood from the mantle, however, returns directly to the auricles without passing through kidneys or gills, but probably freed from its waste none the less. The so-called " organ of Keber " consists of " pericardial glands " on the epithelium of the pericardial cavity. They seem to be connected with excretion. Many of the cells lining the blood channels secrete glycogen, the principal product of the Vertebrate liver. Respiratory system. — Lying between the mantle flaps and the foot there are on each side two large gill-plates, whence the title Lamelhbranch. They are richly ciUated ; their internal structure is like complex treUis-work ; their cavities communicate with the supra-branchial chamber. As in many other Molluscs, the gills or ctenidia are not merely surfaces on which blood is purified by the washing water-currents (a respiratory function), but some of their many cilia waft food-particles to the mouth (a nutritive function), and in the females the outer gill-plate shelters and nourishes the young larvae (a reproductive function). The water may pass through the gills to the supra-branchial chamber and thence out again, or over the gills to the mouth, and thence into the supra-branchial chamber. It is likely that the mantle has no small share in the respiration. In many cases, e.g. Lutraria elliptica, the posterior end of the mantle gives origin to a contractile respiratory siphon, a double tube, the upper half of which is expiratory and the lower half inspiratory. A cross-section shows a cuticular investment of conchin, a layer of epidermis, a narrow zone of circular muscle-fibres, a thick zone of longitudinal STRUCTURE OF FRESH-WATER MUSSEL 453 muscle-fibres, a narrow zone of circular muscle-fibres, an internal epithelium, and the two canals. The white circular muscle-fibres are unstriped ; the longitudinal muscle- fibres, which are greyish yellow, show a lozenge-shaped marking as in the more opaque fibres of the adductor muscles. The precise structure and attachment of the gill-plates is complex, but it is important to understand the following facts : — (a) A cross- section of the two gill-plates on one side has the form of a W, one half of which is the outer, the other the inner gill-plate ; (b) each of these gill-plates consists of a united series of gill filaments, which descend from the centre of the W and then bend up again ; (c) adjacent fila- ments are bound together by fusions and bridges both horizontal and vertical, so that each gill-plate becomes hke a complex piece of basket work ; (d) both gill -plates begin by the downward growth of filaments from a longitudinal " ctenidial axis," the position of which on cross- section is at the median apex of the W ; (^) this mode of origin, and the much less complex gills of other bivalves, lead one to believe that there is on each side one gill consisting of two gill-plates formed from a series of united and reflected gill filaments. On the gills there are often parasitic mites {Unionicola or Atax ypsilophorus). Excretory system. — The paired kidney, which used to be called the " organ of Bojanus," lies beneath the floor of the pericardium. Each half is a nephridium bent upon itself, with the loop posterior, the two ends anterior. The lower part of this bent tube is the true kidney ; it is dark in colour, spongy in texture, and excretes guanin and other nitrogenous waste from the blood which passes through it. It has an internal opening into the pericardium, which thus communicates indirectly with the exterior. The upper part of the bent tube, lying next the floor of the pericardium, is merely a ureter. It conveys waste products from the glandular part to the exterior, and opens anteriorly just under the place where the inner gill-plate is attached to the visceral mass. As already mentioned, the " pericardial glands " probably aid in excretion, and possibly the same may be said of the mantle. The reproductive organs. — These He in the upper part of the foot, adjacent to the digestive gland. Ovaries and testes occur in diflferent animals, and the two sexes are distinguishable, though not very distinctly, by the greater whiteness of the testes and by slight differences in the shells. The females are easily known when the larvae begin to 454 PHYLUM MOLLUSCA accumulate in crowds in the outer gill-plates. The repro- ductive organs are branched and large ; there are no accessory structures ; the genital aperture lies on each side under that of the ureter. The ova pass from the ovaries in the foot, and appear to be moved to the exhalant region, whence, however, they do not escape, but are crowded backward till they pass into Fig. 257. — Glochidium larva of the fresh-water mussel. SH., Valve of the shell; MU., adductor muscle-fibres workmg the valves ; T., a tooth on the margin of the sheU ; B., a thread of glutinous byssus, which effects fixation ; C, a tuft of sensitive cells inside the mantle. The size of the larva is that of a small pin's head. the cavity of the outer gill-plate. At some stage they are fertilised by spermatozoa drawn in by the water currents, though it is difficult to believe that this is entirely a matter of chance. Development takes place within the external gill-plate, and the larvse feed for some time on mucus secreted by the gill. Development and life-history. — The development of Anodonta differs in certain details from that of most bivalves, perhaps in adapta- tion to fresh -water conditions. Moreover, a temporary parasitism of the larva has complicated the later stages. DEVELOPMENT OF ANODONTA 455 The egg-cell is surrounded by a vitelline membrane, and attached to the wall of the ovary by a minute stalk, the insertion of which is marked on the liberated ovum by an aperture or micropyle, through which the spermatozoon enters. Segmentation is total but unequal. A number of small clear yolkless cells are rapidly divided off from a large yolk-containing portion, which is slower in dividing. Eventually a hollow ball of cells or blastosphere results. On the posterior dorsal region a number of large opaque cells form an internally convex plate — the beginning of the future shell-sac. A pah: of large cells are intruded into the central cavity, and begin the mesoderm. On the under-surface posteriorly there is a slight protrusion of ciliated cells forming a cihated disc. In front of this, at an unusually late stage, an invagination estabhshes the archenteron, and the embryo becomes a gastrula. The shell-sac forms an embryonic shell, and many of the mesoderm cells combine in an adductor muscle. The mouth of the gastrula closes, and a definite mouth is subsequently formed by an ectodermic invagina- tion. Gradually a larva peculiar to fresh-water mussels, and known as a Glochidium, is built up (see Fig. 257). The Glochidium has two triangular, delicate, and porous shell valves, each with a spiny incurved tooth on its free edge. The valves clap together by the action of the adductor muscle. The mantle lobes are very small, and their margins bear on each side three or four patches of sensory cells. The foot is not yet developed, but from the position which it will afterwards occupy there hang long attaching threads of " byssus," which moor the larva. If it manages to anchor itself on the tail, fins, or gills of a fish, the Glochidium shuts its valves and fixes itself more securely, and is soon surrounded by a pathological growth of its host's skin. In this parasitic stage a remarkable metamorphosis occurs. The sensory or tactile patches not unnaturally disappear ; the " byssus " and the embryonic " byssus glands " vanish, but a true byssus gland (which remains quite rudimentary in Anodonta) appears ; the single adductor atrophies, and is replaced by two ; the foot and the gills make their appearance ; the embryonic mantle lobes increase greatly, or are replaced by fresh growths ; and the permanent shell begins to be made. After this metamorphosis, when the larva has virtually become a miniature adult, no longer so Hable to be s^wept away, it drops from its temporary host to the bottom of the pond or river pool. Third Type of Mollusca. The Common Cuttlefish {Sepia officinalis), one of the Dibranchiate Cephalopods Habits. — This common cuttlefish is widely distributed, especially in warmer seas like the Mediterranean. Unlike Octopus, which usually lurks passively, Sepia is an active 456 PHYLUM MOLLUSCA swimmer ; it moves head foremost by working the fins which fringe the body, or it jerks itself energetically back- wards by the outgush of water through the funnel. It likes the light, and is sometimes attracted by lanterns. The beautiful colours change according to external conditions and internal emotions ; and a plentiful discharge of ink often covers its retreat from an enemy. Its food includes fish, other molluscs, and crabs. In spring the female attaches her encapsuled eggs to seaweeds and other objects, and often comes fatally near the shore in so doing. The cuttles are caught for food and bait. The " cuttle bone " and the pigment of the ink-bag are sometimes utilised by man. External appearance. — A large Sepia measures about Fig. 258. — Side vie.w of Sepia. — After Jatta. 10 in. in length and 4 to 5 in breadth ; the body, fringed by a fin, is shaped like a shield, the broad end of which bears a narrowed head, with eight short and two long sucker-bearing arms. Besides the diffuse pigment cells, there are bands across the " back." The large eyes, the parrot-beak-like jaws protruding from the mouth, the spout- like funnel on the neck, and the mantle cavity are con- spicuous. Beside the eyes are the small olfactory pits ; within the mantle cavity lie the anus and the openings of the nephridia and genital duct. The true orientation of the different regions in Sepia is not obvious. If the " arms " surrounding the mouth be divided portions of the anterior part of the " foot," the ventral surface is that on which the animal rests when we make it stand on its head. We can fancy how the " foot " STRUCTURE OF SEPIA 457 of a snail might grow forward and surround the mouth, so as to bring that into the middle of the sole. Then the visceral mass has been elongated in an oblique dorso- posterior direction, so that the tip of the shield, directed forward when the cuttle jerks itself away from us, represents in anatomical strictness the dorsal surface tilted backwards. (As above noticed, the animal may also swim with foot and mouth in front.) The side of lighter colour, marked by the mantle cavity and the siphon or funnel, is posterior and slightly ventral ; the banded and more convex side, on which the cerebral ganglia lie in the head region, and on which the shell lies concealed in the visceral region, is anterior and slightly dorsal. Skin. — There are numerous actively changeful pigment cells or chromatophores lying in the connective tissme beneath the epidermis. Each cell is expanded by the contraction of muscular cells which radiate from it, and contracts when these relax. It is probable that these chromatophore cells have some protoplasmic spontaneity of their own, but the controlling muscular elements are also affected by nervous impulses from the central ganglia. As the cells dilate or contract, the pigment is diffused or concentrated, and the colours change. The animal's beauty is further enhanced by numerous " iridocysts " or modified connective tissue cells, with fine markings which cause iridescence. Muscular system. — The cuttlefish is very muscular, notably about the arms, the mantle flap, and the jaws. Many of the muscles show double oblique striping. The animal seizes its prey by throwing out its two long arms, which are often entirely retracted within pouches. With great force it jerks itself backwards by contracting the mantle cavity, and making the water gush out through the pedal funnel. This mode of locomotion is very quaint. At one time the mantle cavity is wide, and you can thrust your fingers into its gape ; when about to contract, this gape is closed by a strange double hook-and-eye arrange- ment ; contraction occurs, and the water, no longer free to leave as it entered, gushes out by the funnel, the base of which is within the mantle cavity. The suckers on the arms are muscular cups, borne on little stalks (unstalked 4S8 PHYLUM MOLLUSCA in Octopus^ etc.), well innervated, and able to grip with a tenacity which in giant cuttlefish is dangerous even to men. The inner edge of the cup margin is supported by a chitinoid ring bearing small teeth. Each cup acts as a sucker, in a fashion which has many analogues, for a retractor muscle increases the size of the cavity after the margin has been applied to some object. The external pressure is then greater than that within the cup, and the little teeth keep the attach- ment from slipping. It seems likely that the arms represent a pro- podium, and the siphon a mesopodium, and a valve within the siphon has been compared to a metapodium. Skeletal system. — An internal skeleton is repre- sented by supporting carti- laginous plates in various parts of the body, especi- ally — (a) in the head, round about the brain, arching over the eyes, enclosing the " ears " ; {b) at the bases of the arms ; {c) as a crescent on the neck ; {d) at the hook-and-eye arrangement of the mantle flap ; {e) along the fringing fins. Ramified " stellate " cells lie in the structure- less transparent matrix of the cartilage. On the shore one often finds the " cuttle bone " or sepio- staire, which is sometimes given to cage birds to peck at for lime, or used for polishing and other purposes. It lies on the front side of the animal, covered over by the mantle sac. In outline it is somewhat ellipsoidal, thinned at the edges like a flint axe-head, and with curved markings which indicate lines of growth. In the very young Sepia it con- sists wholly of the organic basis conchiolin, but to this lime Fig. 259. External appearance of a squid {Loligo). STRUCTURE OF SEPIA 459 sh — i is added from the walls of the sac. Between the plates of lime there is gas, and though the structure may give the cuttle some stability, it is probably of more use as a float. Internal appearance. — When the mantle flap is cut open and reflected, the two plume-like gills are seen, and the lower end of the siphon. The dark outline of the ink-bag, followed along towards the head, leads our eyes to the end Sr of the food canal. Near this are the external aper- tures of the two kidneys and of the genital duct. On each side of the base of the funnel lies a very large and unmistakable " stellate " ganglion. Re- moving the skin as care- fully as possible over the whole visceral region be- tween the gills, and taking precautions not to burst the ink-sac, we see the median heart, the saccular kidneys, contractile struc- tures or branchial hearts at the base of each gill, and the essential repro- ductive organs near the apex of the visceral mass. Disturbing the arrange- ment of these organs, we can follow the food canal, with its stomach, digestive gland, etc. Nervous system. — Three pairs of gangUa surround the gullet — cerebral on the dorsal and anterior side, pedal and pleuro-visceral on the ventral and posterior side (Fig. 260), but lying so close together that their boundaries are defined Fig. 260.- Sepia.- -Diagram of the structure of -Mainly after Pelseneer. , Eight short arms around mouth ; l.a., one of the two long arms ; b., beak of the mouth ; e.g., cerebral ganglia, with com- missures to the others; £., eye ; g., guUet ; d.g., digestive gland (the " salivary glands " are not represented) ; St., stomach ; a., anus ; sh., shell-sac with sepiostaire ; k., kidney ; R., reproductive organ ; br.h., branchial heart ; g., a gill ; i.b., ink-bag ; m.c, mantle cavity ; /., funnel. 460 PHYLUM MOLLUSCA with difficulty. All are well protected by the investing cartilages. The cerebral ganglia are three-lobed, and are connected anteriorly by two commissures with a " supra-pharyngeal " ganglion, which gives off nerves to the mouth and lips, and is connected also with an " infra- pharyngeal " ganglion. The cerebral gangha are also connected by short double commissures with the pedals and pleuro-viscerals on the ventral side of the gullet. The pedal gangha at each side are in part divided into two— one half forming the brachial ganglion which sends nerves to the arrjis, the other the infundibular which supplies the funnel. The following chief nerves arise from the central system : — ■ (i) The very thick optic nerves are given off from the commissures between cerebrals and pleuro-viscerals, and lead to a large optic ganglion at the base of each eye. (2) Ten nerves to the " arms " are given off by the pedal ganghon, and this is one of the reasons which have led most morph- ologists to regard these arms as portions of the " foot." (3) Two large nerves from the more ventral portion of the pleuro- visceral gangha form a visceral loop, and give off many branches to the gills and other organs. From the pleural portion arise two mantle nerves, each of which ends in a large stellate ganglion. Sense organs. — The eyes are large and efficient. They present a striking resemblance to those of Vertebrates, and, as they are not " brain eyes," they illustrate how superficially similar structures may be developed in different ways and in divergent groups. In cuttlefishes the eyes lie on the sides of the head, protected in part by the cartilage surrounding the brain, and in part by cartilages on their own walls. The eye is a sensitive cup arising in great part from the skin. Its internal lining is a complex retina, on the posterior surface of which the nerves from the optic ganglion are distributed. It seems likely that the Cephalopod retina corresponas only to the rods and cones (the sensory part) of the Vertebrate retina. In the cavity of the cup there is a clear vitreous humour. The mouth of the cup is closed by a lens, supported by a " ciliary body." The lens seems to be formed in two parts — an outer and an inner plano-convex lens. The pupil in front of it is fringed by a con- tractile iris. The outer wall of the optic cup is ensheathed by a strong supporting layer — the sclerotic, which is in part strengthened by cartilage, covered by a silvery membrane, and continued into the iris. In front of the eye there is a transparent cornea, and the skin also forms protecting lids. Round about the optic ganglion there is a strange " white body," which seems to be a fatty cushion on which the eye rests. The two ear-sacs, containing a spherical otolith and a fluid, sometimes with calcareous particles, are enclosed in part of the head cartilage, STRUCTURE OF SEPIA 46 1 close to the pedal ganglia. The nerves seem to come from the pedals, but it is said that their fibres can be traced up to the cerebrals. A ciliated " olfactory sac " lies behind each eye, and is innervated from a special ganglion near the optic. There are no osphradia of the usual type. Finally, there are tactile or otherwise sensitive cells on various parts of the body, especially about the arms. Apart from sight altogether, an octopus can find a dead fish at a distance of over a yard in a few minutes, and even slight movements in the water are detected. In many Decapods there are luminous organs, usually on the ventral surface in diverse positions, and often buried. They may serve as recognition-marks or as search-lights. They may be glandular or non-glandular, and those of the second type are often somewhat eye-like, with pigment layer, reflector, lens, and diaphragm, or with some of these structures. Often, however, the luminescence seems to be due to the activity of symbiotic bacteria. Alimentary system. — The cuttlefish eats food which requires tearing and chewing, and this is effected by the chitinous jaws worked by strong muscles, and by the toothed radula moving on a muscular cushion. The mouth lies in the midst of the arms, bordered by a circular lip, and opens into a large pharynx or buccal cavity (cf. the snail). The narrow gullet passes through the ganglionic mass, and leads into the globular stomach, lying near the dorsal end of the body. The stomach is followed by a caecum or pyloric sac, and the intestine curves headwards again, to end far forward in the mantle cavity. There do not seem to be any glands on the walls of the food canal ; the stomach has a hard cuticle ; the digestion which takes place there must therefore be due to the digestive juices of the glandular annexes. Of these the most important is usually called the liver ; it is bilobed, and lies in front of the stomach, attached to the oesophagus. Its two ducts conduct the digestive juice to the region where the stomach, " pyloric sac," and intestine meet ; and thes^ ducts are fringed by numerous vascular and glandular appendages, which are called " pancreatic," and arise from the wall of the un- paired portion of the nephridia. Far forward, in front of the large digestive gland, lie two small white glands on each side of the gullet, with ducts which open into the mouth (cf. the " salivary glands " of the snail). A diastatic ferment has been proved in the salivary secretion of Cephalopods, but that of Octopus has a poisonous, paralys- 462 PHYLUM MOLLUSCA ing effect on the crabs, etc., which are bitten, and also a peptonising action. At the other end of the food canal, the ink-sac, full of black pigment, probably of the nature of waste products, opens into the rectum close to the anus. This ink-sac is a much enlarged anal gland ; for, while most of the bag is made of connective tissue and some muscle fibres, a distinct gland is constricted off at the closed end, and the neck is also glandular. Beside the anus are two pointed papillae. Vascular system. — The blood of Sepia is bluish, owing to the presence of hasmocyanin in the serum ; the blood cells are colourless and amoeboid. The median but some- what oblique ventricle of the heart drives the blood forward and backward to all parts of the body. It reaches the tissues by capillaries, and apparently also by lacunar spaces. The venous blood of the head region is collected in an annular sinus round the basis of the arms, and passes towards the heart by a large vena cava, which divides into two branchial veins, covered by spongy outgrowths of the nephridia. Joined by other vessels from the apical region of the viscera, each branchial vein enters a " branchial heart " at the base of each gill. The branchial heart is contractile, and drives the venous blood through the gills, whence, purified, it returns by two contractile auricles into the ventricle. There are valves preventing back-flow from the ventricle to the auricles, or from the arteries to the ventricle. Beside each branchial heart lies an enigmatical glandular structure known as a " pericardial gland," possibly an excretory or incipiently excretory organ. The course of the blood differs from that in the mussel and snail in this, that none returns to the heart except from the respiratory organs. In the nephridial outgrowths around the branchial veins the interesting parasite Dicyema is found. Respiratory system. — The blood is purified by being exposed on the two feather-like gills which are attached within the water-washed mantle cavity. The water pene- trates them very thoroughly ; the course of the blood is intricate. At the base of the gills there is some glandular tissue, which those impatient with enigmas have credited with blood-making powers. STRUCTURE OF SEPIA 463 Excretory system.— The excretory system is difficult to dissect and to explain. On each side of the anus there is a little papilla, through which uric acid and other waste products ooze out into the mantle cavity, and so into the water. A bristle inserted into either of these two papilla leads into a large sac— the nephridial sac. But the two sacs are united by two bridges, and they give off an unpaired dorsal elongation, which extends as far back as the reproductive organs. The dorsal wall of each nephridial sac becomes intimately associated with the branchial veins, and follows their outlines faithfully. It is likely that waste material passes from the blood through the spongy appendices into the nephridial sacs. pjf;_ 261. — Diagram of circulatory and excretory systems in a Decapod like Sepia. — After Pelseneer. I Gill • 2, renal sac ; 3, afferent branchial vessel ; 4, branchial heart ; ' 5 'abdommal vein ; 6, heart ; 7, viscero-pericardial sac (body cavity) ; 8, genital organ ; 9, posterior aorta ; 10, " auricle " ; 11, o^landular appendix of branchial heart ; 12, renal appendices of branchial vein ; 13, external aperture of kidney ; 14, vena cava ; 15, anterior aorta ; 16, bifurcation of vena cava ; 17 reno- pericardial aperture. Into the terminal portion of each nephridial sac, a little below its aperture at the urinary papilla, there opens by a cihated funnel another sac, which is virtuallv the body cavity. It surrounds the heart and other organs, and is often called the viscero-pericardial cavity. Through the kidneys or nephridial sacs it is in communication with the exterior. Associated with the branchial hearts there are numerous diminutive cells which contain ammoniacal salts, phosphates, pigment, etc. ; these waste products are probably passed into the blood and got rid of by the kidneys, just as, in a Vertebrate, the urea formed in the liver passes by the blood to the kidneys. In Invertebrates there is often this co-operation between " closed kidneys " and " open kidneys." 464 PHYLUM MOLLUSCA Reproductive system. — The sexes are separate, but there is not much external difference between them, though the males are usually smaller, less rounded dorsally, and have slightly longer arms. When mature, the male is easily known by a strange modification on his fifth left arm. The essential reproductive organs are unpaired, and lie in the body cavity towards the apex of the visceral mass. The testis — an oval yellowish organ — lies freely in a peritoneal sac, near the apex of the visceral mass. From this sac the spermatozoa pass along a closely twisted duct — the vas deferens. This expands into a twofold " seminal vesicle," and gives off two blind outgrowths, of which one is called the " prostate." The physiological interest of these parts is that within them the spermatozoa begin to be arranged in packets. In this form they are found within the next region, the spermatophore sac, which opens to the exterior to the left of the anus. Each spermatophore is like an automatically explosive bomb ; within the transparent shell there lies a bag of spermatozoa, and a complex spring-like arrangement. Even on the scalpel or slide these strange but efficient bombs will explode. The liber- ated spermatozoa are of the usual type. The ovary — a large, rounded white organ — lies freely in a peritoneal sac near the apex of the visceral mass. From this sac the eggs pass along a short direct oviduct, which opens into the mantle cavity to the left of the anus. Associated with the oviduct, and pouring viscid secretion into it, are two large " nida- mental glands," of foliated structure. Fig. 262. — Male of Argo- nauta (after Jatta), show- ing " hectocotylus " arm ; compare Fig. 9 of female. Close beside these are accessory glands, of a reddish or yellowish colour, with a median and two lateral lobes ; while at the very end of the oviduct are two more glands. All seem to contribute to the external equipment of the egg. The spermatophores pass from the genital duct of the male to the fifth left arm, which becomes covered with them and quaintly modified. This modification of one of the arms is usual among cuttlefish ; indeed, in some, e.g. Argonaiita and Tremoctopus, the modified arm, with its load of spermatozoa, is discharged bodily into the mantle cavity of the female. There its discoverers described it as a parasitic worm, MOLLUSCAN CHARACTERS 46! " Hectocoiyhis." The lost arm is afterwards regenerated, la Sepia, however, the modified arm is not discharged, but is simply thrust into the mantle cavity of the female. The spermatophores probably enter the oviduct, and burst there. Fig. 263. — Bunch of Sepia eggs attached to plant. — After Jatta. The eggs, when laid, are enclosed within separate black capsules containing gelatinous stuff, but the stalks of the capsules are united, so that a bunch of " sea-grapes " results. General Notes on Molluscs From the description of these three types a general idea of the structure of MoUusca may be obtained, but it should be noted — (i) that all the three types are specialised ; (2) that two small classes, the Solenogastres and the Scapho- poda, are unrepresented in the descriptions ; (3) that in the three classes to which the types belong there is much diversity of structure, this being especially true of the large and heterogeneous class of Gasteropods. In surveying the structure of the whole group, it is con- venient to begin with the most striking of the external characters — the absence or presence of a well-developed head region. In the Lamellibranchs or Pelecypoda the head is absent, and along with it the tentacles, the radula, and the pharynx with all its associated structures. Elsewhere a head 30 466 PHYLUM MOLLUSCA region, usually furnished with tentacles and eyes, and con- taining within it a pharynx and radula, is always present. Best developed in Gasteropods and Cephalopods, the head region may elsewhere be represented, as in DentaUum, merely by a buccal tube fringed with tentacles. Apart from Lamellibranchs, the radula is characteristic and, with few exceptions, universal. Almost as important is the condition of the characteristic Molluscan foot. Primitively this had the form of a ventral creeping sole, as shown, for example, in its simplest condition, in Chiton (Fig. 270). This condition is retained in many Gasteropods, and in the simplest Lamellibranchs, Fig. 264. — Common buckie {Buccinum undatum). e., Eye ; s., respiratory siphon ; o., operculum ; /., foot. like Solenomya. In most Lamellibranchs, however, in adaptation to a more or less passive life in the sand, the foot became wedge-shaped, and the characteristic byssus gland, which secretes attaching threads, was developed. In the Cephalopods the foot became greatly modified, and in those related to Sepia a portion of it is specialised as the funnel — the main organ of active locomotion. That the condition of the foot cannot in itself be employed as a basis of classification is, however, obvious, when its differences within the limits of a class are considered. Thus it is obsolete in the pelagic Phyllirhoe among Gasteropods, in the sedentary oyster among Lamellibranchs ; in the pelagic Pteropods part of it forms lateral wing-like lobes used in MANTLE IN MOLLUSCS 467 swimming, while in lanthina, which has a similar habit, its chief use is to secrete a " float " to which the egg-capsules are attached. In various Lamellibranchs, and in Den- talium, it is modified as a conical boring organ. The mantle is another important Molluscan structure, and as it secretes the shell, the shape of the latter is of course determined by it. Primitively the mantle is repre- sented by a uniform downgrowth of skin from the dorsal surface, surrounding the ventral foot, and secreting a dorsal cap-shaped shell. Such a simple condition occurs in the Umpet. In the Lamellibranchs, with the lateral flattening of the body, the mantle becomes divided into right and left halves, and the shell becomes two-valved. In most Lamelli- FiG. 265. — Bivalve (Panopcea norvegica), showing siphons. e., Exhalant aperture ; ('., inhalant aperture. branchs the mantle is prolonged into two tubes or siphons, through which the water of respiration enters and leaves the mantle cavity. A similar but unpaired siphon is found in many Gasteropods. In Scaphopoda the mantle folds fuse ventrally to form a continuous tube. In most Gasteropods the mantle skirt is retained, and secretes a spiral shell, as well as enclosing a space in which the gills lie ; in some, both mantle and shell are absent. ^ In the snail and its allies (P.ulmonata), the mantle forms the pulmonary chamber, which opens to the exterior by a small aperture. In Cephalopoda the mantle skirt is well developed and muscular, and, besides sheltering the gills, is of much importance in locomotion. Typically the Mollusca are bilaterally symmetrical animals, and this symmetry is marked in the Solenogastres and Lamellibranchiata, and occurs to a less extent in the 468 PHYLUM MOLLUSCA Cephalopoda (cf. the unpaired genital organs). In most Gasteropoda it is completely lost. This seems to be in some way associated with the dorsal displacement of the viscera in Gasteropods to form the (usually coiled) visceral hump. In Cephalopods there is a somewhat similar dis- placement in a postero-dorsal direction, in Lamellibranchs in a ventral direction, but in neither case is it so marked as in Gasteropods. The characters of the internal organs of MoUusca must be inferred from the description of the types, but the nature of the respiratory organs may be briefly noted. Typically, these consist of two feathery gills, or ctenidia, with an axis attached to the body and bearing a double row of lamellae. ^#SI^ s^-^^ ',m>!^^ Fig. 266. — Nudioranch {Dendronotus arborescens), showing dorsal outgrowths forming adaptive gills. These are sheltered beneath the mantle, and bear at their bases two osphradia or smelling patches. Gills of this typical form occur in Cuttles {Nautilus has four), in the simplest Gasteropods (but many other Gasteropods have a simple unpaired gill), and in the lowest Lamellibranchs {Solenomya, Nucula, etc.). The respiratory organs in other Mollusca show much diversity when compared with this primitive type. Thus the gills may be totally suppressed and the mantle may directly take on a respiratory function. This occurs in many marine Gasteropods, for example, in the common limpet {Patella) (Fig. 267), as well as in terrestrial forms like the snail, where the mantle cavity forms the pulmonary chamber. Even in Lamellibranchs, where the gills are present in much modified form, it is SHELLS OF MOLLUSCS 469 probable that the mantle has much importance in respira- tion, the gills being perhaps of most importance in connec- tion with nutrition, and as brood-chambers. In those Gasteropods in which the gills are suppressed, there are often special respiratory organs (" adaptive gills "), such as the circle of plumes around the anus in Doris and its allies (Fig. 266). The osphradia are absent in Cephalopods, except in Nautilus, and one at least is usually suppressed in Gasteropods. Shell. — On the dorsal surface of almost every mollusc embryo there is a little shell- sac in which an embryonic shell is begun ; the adult shell, however, is always started and increased by the mantle. Like other cuticular products, it has an organic basis (conchiolin or conchin), along with which carbonate of lime is associ- ated. There is a thin outer '* horny " layer, a thick median " prismatic " stratum of hme, and an internal mother-of-pearl layer, which may be divided into two strata by a clear intermediate layer, well seen in the fresh-water mussel, Margaritana margaritifera. Pearls are formed in sacs of the external epithelium of the mantle, sometimes aromid a centre of a periostracum-like substance, sometimes around the larva of a Trematode or Cestode. They are to be dis- tinguished from concretions formed around an intruded irritant particle. The latter do not show the characteristic lamination of pearls. Some pearl-like structures are fixed to the shell ; true pearls are free. While some investigators insist on the parasitic origin of pearls, others are equally emphatic in declaring that they may arise independently. But all are agreed that they are pathological products. Larvae. — In their life-history m-ost IMolluscs pass through two larval stages. The first of these is a pear- shaped or barrel-shaped form, with a curved gut, and with a ring of cilia in front of the mouth. It is a " trocho- FiG. 267. — Ventral surface of Patella vulgata. — After Forbes and Hanley. Note simple eyes at base of tentacles, mouth, median foot, and vascular margin of mantle replacing the absent giUs. 470 PHYLUM MOLLUSCA Sphere," such as that occurring in the development of many " worms." Soon, however, the trochosphere grows into a yet more efficiently locomotor form — -the veliger. Its head bears a ciliated area or " velum," often produced into retractile lobes ; its body already shows the beginning of " foot " and mantle ; on the dorsal surface lies the little embryonic shell gland (Fig. 248). But although trochosphere and veliger occur in the development of most forms, they do not in any of the three types which we have particularly described — not in Anodonta, partly because it is a fresh-water animal, with a peculiarly adhesive larva of its own ; not in Helix, partly because it is terrestrial ; and not in Sepia, partly because the eggs are rich in yolk. CLASSIFICATION OF MOLLUSCA Leaving aside the difficult Solenogastres, which may not be Molluscs at all, we may rank as lowest the Isopleura, bilaterally symmetrical Gasteropods with many primitive characters. Some of these forms, like Chiton^ are probably not far removed from the primitive Mollusca. From primitive forms, related perhaps to Chiton, Mollusca have diverged in two directions. In Gasteropoda, Scaphopoda, and Cephalopods, the head region becomes well developed, and the radula present in the primitive Isopleura is re- tained, except in rare cases, such as one of the species of Eulima, a semi-parasite. These three classes are therefore often placed together as Glossophora or Odontophora, in contrast to the Lamellibranchiata (Lipocephala or Acephala), where the radula has disappeared, and the head region remains undeveloped. As already seen, however, the lowest Lamellibranchs have a flattened creeping foot and simple feathery gills > in these respects resembling Gasteropods. There is also much reason to believe that the Scaphopoda arose from a stem common to them and the lowest Gasteropods, which are central unspecialised forms. The Cephalopoda are the most highly specialised of all the Mollusca, and in their existing forms at least not nearly related to the other classes. CLASSIFICATION OF MOLLUSCS 471 er Pretre. Class I. Gasteropoda .Molluscs with a usually well-developed head region with tentacles and odontophore. The foot is usually a flat median sole on which the animal creeps ; it is often divided into pro-, meso-, and meta-podium. Most are unsymmetrical, but there is a primitive bilateral symmetry in Isopleura and a secondary superficial bilateral symmetry in some pelagic forms such as Heteropods . The mantle or covering of the visceral sac usually forms a well-marked fold or flap where the visceral sac joins the head fig. 268. Chiton. Aft and foot, and thus encloses a mantle cavity. In most cases the shell is a single piece ; in Chitons there are eight pieces ; in many cases the shell is rudimentary or absent. There is usually a trochosphere and veliger larva, except in terrestrial forms. Sub-class I. Gasteropoda Isopleura The Isopleura are marine Gastero- pods more or less elongated in form, with bilateral symmetry. The sym- metry is not only seen in the form of the body, but in the numerous ctenidia, the paired nephridia, auricles, and genital ducts. The shell consists of . eight pieces. The mouth is anterior ; the anal and nephridial apertures are posterior. The mantle, which bears cuticular spicules, covers at least a great part of the body. The nervous system consists of a cerebral com- missure and two paired longitudinal cords (pedal and visceral), with Fig. 269. — Dorsal view of nervous system of Acan- thochiton. — After Pel- seneer. I, Upper buccal commissure; 2, upper buccal ganglion ; 3, stomatogastric commissure ; 4, labial commissure; 5, sub- radular ganglia ; 6, anterior pedal commissure ; 7, pedal nerve with palliopedal con- nections ; 8, supra-rectal pallial commissure ; 9, pallial nerve ; 10, anastomosis of branches of pedal nerves ; 1 1 , stomatogastric ganglia; 12, oesophageal nerves; 13, cere- bral commissure. 472 PHYLUM MOLLUSCA ganglionic cells but at most very slightly developed ganglia, which run the whole length of the body. Of these paired cords the pedals are connected by numerous cross-commissures , and the viscerals or pallials are united posteriorly by a commissure above the rectum. The bilateral symmetry is shown internally, e.g. in the paired nephridia, auricles, and genital ducts. The class is of ancient origin, dating from the Silurian. There is one order — Polyplacophora, e.g. Chiton. The Isopleiira or Polyplacophora are represented on British coasts by several species of Chiton, sluggish, usually vegetarian, animals, Fig. 270. — Anatomy of Chiton. Ventral surface (after Cuvier). B, Dorsal view of alimentary canal (after Lankester). C, Genital and excretory organs from dorsal surface (after Lang and Haller, diagrammatic), m., Mouth ; a., anus; br., numerous simple gills; /., foot ; b., buccal mass; /., liver; i., intestine; ao., aorta; v., ventricle of heart; r.a. and La., right and left auricles; ov., ovary; od., oviduct; od' ., opening of oviduct ; n., part of nephridium, represented in black throughout ; no., external opening of nephridium ; p., outline of pericardium. occurring from the shore to great depths. The foot is generally as long as the body ; the mantle covers the back and bears eight shell- plates (Fig. 268), perforated, in many cases at least, by numerous sensory organs, which are in part optic ; numerous gills lie in a regular row along a groove on each side between the mantle and the foot. In most cases the eight shell-plates are jointed on one another, and the animal can roll itself up. The uncovered parts of the mantle bear spicules. Ganglia, in the strict sense, are scarcely developed, but there is a supra-oesophageal ganglionic commissure from which the visceral and pedal cords extend backwards along the whole length of the body. There are no special sense organs on the head, which is but slightly differentiated ; but the pallial sense organs are usually numerous and varied. A twisted gut runs through the body, sur- CLASSIFICATION OF MOLLUSCS 473 rounded bv a diffuse digestive gland. There is a radula in the mouth. The heart is median and posterior, and consists of a ventricle and two to eight auricles. There are two symmetrical nephridia opening posteriorly, and consisting of much-branched tubes. The sexes are separate ; a single reproductive organ extends dorsally between gut and aorta almost the whole length of the body ; the genital ducts are paired and open posteriorly in front of the excretory apertures. The ova, with chitinous spiny shells, are usually retained for some time by the female between the mantle and the gills. The segmentation is holoblastic, and a gastrula is formed by invagination. Sub-class 11. Gasteropoda Anisopleura, e.g. Snail, Whelk, Limpet In these more or less asymmetrical Gasteropods, the head region, which is well developed, remains symmetrical, and so does the foot, which is typically a flat creeping organ. But the visceral mass or hump, with its mantle fold, is more or less twisted forwards and to the right. Thus the pallial, anal, nephridial, and genital apertures usually lie on the right side, more or less anteriorly. A further asymmetry is shown by the twisting of the morphologically right gill to the left side, while the original left gill is usually lost. Similarly, one of the nephridia, probably that which is morphologically the left, tends to disappear, and in most cases only one persists — topographically on the left side. The main torsion must be distinguished from the spiral twisting which the visceral hump often exhibits, and from the frequently associated spiral coiling of the univalve shell. Moreover, a superficial secondary bilateral symmetry tends to be acquired by free-swimming forms, e.g. Heteropods. There are never more than two gills of the ctenidium type. The shell is usually in one piece ; but It is sometimes rudimentary or absent. The foot usually contains a mucus gland, and tends to be divided into three regions — the pro-, meso-, and meta-podium. There is a single reproductive organ and genital duct. Branch A. Streptoneura In the torsion of the body one limb of the visceral loop crosses the other in a figure 8. Order i. Zygobranchiata The atrophy of the primitively left -side gills and nephridia is not carried out, or only partially, e.g. Haliotis (ear-shell) ; Fissurella (key- hole limpet) ; Patella (limpet). 474 PHYLUM MOLLUSCA Order 2. Azygobranchiata The originally left gill and the originally left nephridium have been lost. Heart with single auricle ; one gill, one nephridium ; operculum present. Periwinkle {Littorina), buckie (Buccinum, Fig. 264), dog-whelk {Purpura), lanihina, and the majority of the marine Gasteropods with coiled shells, together with some fresh-water forms. The pelagic Heteropods are also included here -.—Atlanta, shell well developed ; Carinaria, with small shell ; Pterotrachea, with no shell. Branch B. Euthyneura The visceral loop does not share in the torsion of the visceral hump. Order 3. Opisthobranchiata The visceral loop is euthyneural, as in snails ; the single auricle lies behind the ventricle ; the shell and mantle are often absent. A. Tectibranchiata. A shell is present, but may be rudimentary ; there is a well-developed mantle fold and a single gill, e.g. Bulla, Aplysia, Dolahella, Umbrella. The Tectibranchiata also include the Pteropoda, the winged snails or sea-butter- fiies, which have become much modified for pelagic life. They have a secondarily acquired bilateral symmetry, and swim by two large lateral lobes of the foot. They often swim actively in shoals, and occur in all seas. They afford food for whales, etc., and the shells of some are abundant in the ooze. They include — (a) Thecosomata, with mantle fold and shell, diet of minute animal or vegetable organisms, closely related to Bulla and its allies. Examples. — Hyalea, Cymhulia. {b) Gymnosomata, without mantle fold or shell in the adult. Closely allied to Aplysia and its allies. Actively carnivorous, e.g. Clio, Pneumoderma. B. Nudibranchiata. Shell, mantle fold, and true gill are absent : various forms of " adaptive gills " may be present, or there may be no special respiratory organs, e.g. sea-slugs, Doris, Eolis, Dendronotus (Fig. 266). Order 4. Pulmonata The visceral loop is short and untwisted, gills are absent, and the mantle cavity functions as a lung ; all are hermaphrodite, e.g. the snail (Helix); the grey slug (L:w/a;i;) ; the black slug (.4 rjow) ; fresh- water snails, such as Limnaa, Planorbis, and Ancylus. Mode of life. — From the number of diverse types which the class includes, it is evident that few general statements MOVEMENTS OF GASTEROPODS 475 can be made about the life of Gasteropods. We are safe in saying, however, that though the majority are sluggish when compared with Cephalopods, they are active when com- pared with Lamellibranchs. The locomotion effected by the contractions of the muscular foot is usually a leisurely creeping, but there are many gradations between the activity of Heteropods in Fig. 271. — -A Pteropod [Cymbulia peronii), showing the wmg-Hke expansions (pteropodial lobes) of the mid-foot - Open sea, the gliding of fresh-watef snails {Limncea) foot upwards across the surface of the pool, the explorations of the periwinkles on the sand of the shore, and the extreme passivity of limpets {Patella), which move only for short distances at a time from their resting-places on the rocks. The number of terrestrial snails and slugs, breathing the air directly by means of a pulmonary chamber, is estimated at over 6000 living species, while the aquatic Gasteropods are reckoned at about 10,000, most of which are marine. 476 PHYLUM MOLLUSCA Of this myriad, about 9000 are streptoneural, the relatively small minority are euthyneural Opisthobranchs and Nudi- branchs, with light shells or none. The Heteropods and some Opisthobranchs live in the open sea ; the great majority of aquatic Gasteropods frequent the shore and the sea bottom at relatively slight depths ; the deep-sea forms are com- paratively few. Gasteropods rarely feed at such a low level as bivalves do— indeed, some of them are fond of eating bivalves. Most Prosobranchs (strep- toneural), with a respiratory siphon and a shell notch in which this lies, are carnivorous, e.g. the buckles (Biiccinum) and " dog-whelks " (Pur- pura) ; on the other hand, those without this siphon, and with an unnotched shell mouth, feed on plants, e.g. the seaweed-eating peri- winkles {Littorina). Most land snails and slugs are vegetarian. Many Gasteropods, both marine and terres- FiG. 272.— Stages in ^^i^^' ^^^ voracious and indiscriminate molluscan development, in their meals ; others are as markedly A, Biastuia of limpet (after Specialists or cpicurcs. Somc marine Patten). B, Gastruia of forms partial to Echinodcrms have a Paludina vivipara (after ,. ^ • r j-i .^ 1 u : , Tonniges) ; v., beginning of salivary sccretiou ot dilutc suiphuric velum; arc, archenteroii ; ^^J^ which chaUgeS the CarbouatC of m., mesoderm cells. C, ' . ^^ . . , Later stage of the same; i;., limC lU the Starhsh intO the morC guSto;,.,'"'»,r'"arr,'; brittle and readily pulverised sul- teron; a., anus ; /..begin- phate. About tcu geucra are para- ning of foot ; sh.g., shell ^. . • t^ i • j gland. sitic ou or m Echmoderms, e.g. Sty lifer, Tiirtonia, Thyca, and the extremely degenerate Entoconcha, within the Holothurian Synapta. Some species of Eulima also live a semi-parasitic life on certain Echinoderms. Life-history. — The eggs of Gasteropods are usually small, without much yolk, but surrounded by a jelly, the surface of which often hardens. In the snail and some others there is an egg-shell of lime. LIFE-HISTORY OF GASPEROPODS 477 Sexual union occurs between hermaphrodites as well as between separate sexes, and fertilisation is effected inside the genital duct. Development sometimes proceeds within the parent, but in most cases the fertilised eggs are laid in gelatinous clumps, or within special capsules. The free- swimming lanthina carries the eggs in capsules attached to a large raft-like float towed by the foot. On the shore one often finds numerous egg-capsules of the " buckie " {Biiccinum iindatum) united in a ball about the size of an orange. Under the ledges of rock are many little vases or cups, the egg-capsules of the dog- whelk {Purpura lapillus). In the buckie and whelk, and in some other forms, there is a struggle for existence — an infant cannibalism — in the cradle, for out of the numerous embryos in each capsule only a few reach maturity— those that get the start eating the others as they develop. The development is usually simple and typical. In other words, segmentation is total though often unequal ; gastru- lation is embolic or epibolic according to the amount of yolk present ; the gastrula becomes a trochosphere, and later a veliger (Fig. 272, C). Past history.— As the earth has grown older the Gasteropods have increased m numbers. A few have been disinterred from the Cambrian rocks ; thence onwards they increase. Most of the Palaeozoic genera are now quite extinct, but many modern famihes trace their genealogy to the Cretaceous period. Those with respiratory siphons were hardly, if at all, represented in Palaeozoic ages, and the terrestrial air-breathers are comparatively modern. Ecology. — As voracious animals, with irresistible raspers, Gasteropods commit many atrocities in the struggle for existence, and decimate many plants. Pro- fessor Stahl shows, however, that there are more than a dozen different ways in which plants are saved from snails —by crystals, acids, ferments, etc. ; in short, by consti- tutional characteristics sufficiently important to determine survival in the course of natural selection or ehmination. As food and bait, many Gasteropods are very useful ; their shells have supplied tools and utensils and objects of delight ; the juices of Purpura and Murex furnished the Tyrian purple, more charming than all aniline. 478 PHYLUM MOLLUSCA Class II. SOLF.NOGASTRES The members of this class are worm-like animals, in which the mantle envelops the whole body and bears numerous spicules, but no shell. It is somewhat doubtful if they are Molluscs at all. There are two families — Neomeiiiidae and Chstodermidae. Of Neomeniidfe, six genera are known, e.g. Neomenia and Pro- neomenia. They have a longitudinal pedal groove, an intestine without distinct digestive gland, two neph- ridia with a common aperture^ and hermaphrodite reproductive organs. The Chaetodermida,', represented by one genus Chcetoderma, are cylin- drical in form, without a pedal groove, with a radula bearing one tooth, with a distinct digestive gland, and with two nephridia opening separately into a posterior cavity, which also contains two gills. The sexes are separate. Class III. SCAPHOPODA Very different in many respects from Gasteropoda are the Scapho- poda, of which Dentalium (Ele- phant's tooth-shell) is the com- monest genus. They are apparently related to the Zygobranchiate Gas- teropods, and also to the simplest Bivalves. They burrow in the sand at considerable depth off the coasts of many countries. The mantle has originally two folds, which fuse ventrally, and the shell becomes cylindrical, like an elephant's tusk. It is open at both ends. The larger opening (directed downwards in the sand) is anterior, the concave side of the shell is dorsal. The mouth opens at the end of a short buccal tube, at the base of which is a circle of ciliated tentacles. The foot is long, with three small terminal lobes. It is used in slow creeping, and is protruded at the anterior opening. There are cerebral and pleural ganglia near one another in the head, pedal ganglia in the foot, and a long untwisted visceral loop with olfactory ganglia near the posterior anus. Sense organs are represented by otocysts beside the pedal ganglia. There is an odontophore with a simple radula. The food consists of minute animals. There is a much reduced heart, and Fig. 273. — Proneomenia. Ner- vous system. — From Hubrecht. e.g.. Cerebral ganglia ; s/g., sublingual ; a.p.g., anterior pedal ; p.p-g., pos- terior pedal ; p.v.g., posterior vis- cerals ; si., sublingual connectives ; cpc, cerebro-pedal connective ; pe., longitudinal pedal nerves ; la., longi- tudinal lateral nerves. BIVALVE MOLLUSCS 479 colourless blood circulates in the body cavity. There are two nephridial apertures, one on each side of the anus ; and two nephridia. The sexes are separate ; the reproductive organ is simple and dorsal in position ; the elements pass out by the right nephridium. The gastrula is succeeded by a free-swimming stage, in which there is a hint of a velum and a rudimentary shell gland. Examples. — Dentalium, Enialium. About forty widely distributed species are known. Dentalium entale occurs off British coasts. The genus occurs as a fossil from Devonian strata onward. Class IV. Lamellibranchiata or Bivalves {Synonyms — Acephala, Conchifera, Pelecypoda, Lipocephala, etc.) Examples. — Cockles, Mussels, Clams, and Oysters Lamellihranchs are bilaterally symmetrical Molluscs, in which the body is compressed from side to side and the foot more or less ploughshare-like. The head {or prostomium) region remains undeveloped, and without tentacles ; radula, horny jaws, and salivary glands are absent, but there is a pair of labial palps on each side of the mouth. The mantle "skirt is divided into two flaps, which secrete the two valves of the shell, now lateral instead of dorsal in position. The valves are united by a dorsal elastic ligament, and closed by two transverse adductor muscles or by one. Internal bilciteral symmetry is marked by the paired nature and disposition of the nephridia, auricles, gills, digestive gland, and reproductive organs. The gills {ctenidia) consist of numerous gill filaments , which typically grow together into large plates {hence the title Lamellibranch). There are usually three pairs of ganglia : (a) cerebro-pleurals in the head ; (b) pedals in the foot ; (c) viscerals at the posterior end of the body. The heart consists of a ventricle and two auricles, and is surrounded by a pericardium which is coelomic in origin, and communicates with the exterior by means of the two nephridia. Repro- ductive organs are always simple, and the sexes are usually separate. The typical development includes trochosphere and veliger stages. Most Lamellibranchs feed on microscopic organisms and particles ; the distribution is very wide, both in salt and fresh water ; the general habit is sedentary or sluggish. 480 PHYLUM MOLLUSCA Classification. — That of Pelseneer is based on the structure of the gills. Order i. Protobranchia. — There are two simple posterior gills, quite similar to those of Zygobranchs ; the foot has a flattened creeping surface : the pleural and cerebral gangha are distinct, e.g. Nucula, Solenomya. Order 2. Filibranchia. — -The gill filaments are greatly elongated and reflected, so that they consist of an ascending and a descending limb, e.g. Area (Noah's-ark shell), Mytilus (edible mussel), Modiola (horse-mussel). Order 3. Pseudo-Lamellibranchia. — The successive gill filaments are loosely connected together to form gill-plates, e.g. Pecten (scallop), Ostrea (oyster). Order 4. Eulamei.libranchia. — The separate filaments are no longer discernible ; the gills form double flattened plates. The great majority of Bivalves are included here, e.g. Anodonta, Venus, Pholas (a boring form), My a. General Notes on Lamellibranchs Structure. — The organs which show most variety in bivalves are the foot, the gills, the adductor muscles, and the mantle skirt. The foot shows much diversity in size and shape ; the pedal gland of Gasteropods is often represented by a " byssus " gland, which secretes attaching threads, well seen in the edible mussel {Mytilus). The gills show a series of gradations, from a slight interlocking of separate gill filaments to the formation, by complicated processes of " concrescence," of plate-like structures such as those of Anodonta. These processes are more closely related to the method of nutrition than of respiration, which, indeed, is probably largely performed by the mantle skirt. The mantle skirt is often united to a greater or less extent inferiorly, and is often prolonged and specialised posteriorly to form exhalant and inhalant "siphons" (Fig. 265). These siphons sometimes attain a considerable length ; they occur especially in forms such as Mya, which live buried in sand or mud, or which burrow in wood or stone, e.g. Pholas. The diversities in the adductor muscles afford one basis for classification. We may associate with the sluggish habits and sedentary Hfe of bivalves— (i) the undeveloped state of the head region ; (2) the large- ness of the plate-like gills, which waft food-particles to the mouth ; and (3) the thick limy shells. We may reasonably associate these and other facts of structure {e.g. the rarity of anterior eyes, biting or rasping organs) with the conditions of life. In some LameUibranchs, e.g. Mytilidaj, small eyes occur at the base of the most anterior filament of the inner gill-plate ; in some other cases they are present in the larva, but not in the adult. Habit.— Most bivalves, as every one knows, live in the sea, and their range extends from the sand of the shore to great depths. They occur in all parts of the world, though only a few forms, like the edible mussel {Mytilus edulis), can be called cosmopolitan. Some, such as LIFE-HISTORY OF BIVALVES 48 1 oysters, can be accustomed to brackish water. The fresh-water forms may have found that habitat in two ways — {a) a few may have crept slowly up from estuary to river, from river to lake ; Dreissensia poly- ■morpha has been carried on the bottom of ships from the Black Sea to the rivers and canals of Northern Europe ; and it is likely that aquatic birds have assisted in distributing little bivalves like Cyclas ; {b) on the other hand, it is more probable that the fresh-water mussels {Unto, Anodonta, etc.) are relics of a fauna which inhabited former inland seas, of which some lakes are the freshened residues. Between the active Lima and Pecten, which swim by moving their shell valves and mantle flaps, and the entirely quiescent oyster, which has virtually no foot, there are many degrees of passivity, but most incline towards the oyster's habit. Of course, there is much internal activity, especially of cihated cells, even in the most obviously sluggish. The cockle (Cardium) uses its bent foot to take small jumps on the sand ; the razor -fish [Solen) not only bores in the sand, but may swim backwards by squirting out water from within the mantle cavity ; many {e.g. Teredo, Pholas, Lithodomus, Xylophaga) bore holes in stone or wood ; in the great majority the foot is used for slow creeping motion. The food consists of Diatoms and other Algae, Infusorians and other Protozoa, minute Crustaceans and organic particles, which the ciHa of the gills and palps sweep towards the mouth. The bivalves are them- selves eaten by worms, starfishes, gasteropods, fishes, birds, and even mammals. Several commensal bivalves (Montacutidae) are known — Montacuta on heart -urchins, Entovalva in the gullet of Synaptids, Scioberetia on a sea-urchin, and J ousseaumiella on a Sipunculid. Life-history. — The eggs are sometimes laid in the water, either freely or in attached capsules, or they are fertihsed by spermatozoa drawn in with the inhaled water, and are subsequently sheltered within the body during part of the development. In the Unionidae the embryos are retained within the cavities of the outer gills ; in Cyclas and Pisidimn there are special brood-chambers at the base of the gills. In Cyclas the embr^^os are nourished by the maternal epithehal cells. Segmentation is always unequal ; a gastrula may be formed by invagina- tion or by overgrowth, the two cases being connected by a series of gradations. A trochosphere stage is more or less clearly indicated, being most obvious in cases where the eggs are laid in the water. The free-swimming trochosphere becomes a veliger, and this is modified into the adult. The fresh-water forms, with the exception of Dreis- sensia polymorpha, in which the habit is recently acquired, do not possess free-swimming larva^ ; this must be regarded as an adaptation. Past history of bivalves. — Even in Cambrian rocks, which we may call the second oldest, a few bivalves have been discovered ; in the Upper Silurian they become abundant, and never fall off in numbers. Those with one closing muscle to the shell seem to have appeared after those which have two such muscles. Those which, from the shell markings, seem to have had an extension of the mantle into a pro- trusible tube or siphon, were also of later origin. The present fresh- water forms were late of appearing. Of all the fossil forms the most 31 482 PHYLUM MOLLUSCA remarkable are large twisted shells, called Hippuntes (Rudistae), whose remains are often very abundant in deposits of the chalk period. Class V. Cephalopoda. Cuttlefish Examples. — Sepia, Octopus (Polypus), Loligo, Nautilus The Cephalopods are bilaterally symmetrical and free- swimming. The head is surrounded by numerous *' arms " bearing tentacles or suckers. These arms seem to be equivalent to processes of the margin of the foot. Another portion of the foot forms a partial or complete tube— the " siphon " or ''funnel " — through which water is forcibly expelled from the mantle cavity. The muscular mantle flap which shelters well-developed plumose gills is posterior in position ; the visceral hump shows no trace of spiral coiling, but is elongated in a direction anatomically dorsal and posterior, though it may point forwards when the animal propels itself through the water. Except in the Pearly Nautilus, the shell of modern forms has been enclosed by the mantle, and is, in most cases, only hinted at. There is a very distinct head region, furnished with eyes and other sensory structures, and the mouth has strong beak-like jaws, as also a well-developed radula. The nervous system shows considerable specialisa- tion ; the chief ganglia are concentrated in the head, and sheltered by cartilage. The true body cavity, pericardium of other Molluscs, is usually well developed, and frequently surrounds the chief organs. Except in the Nautilus, it com- municates with the exterior by the nephridia. The nephridia are disposed on the walls of the afferent branchials. The vascular system is well developed, and, except in the Nautilus, there are accessory branchial hearts. The sexes are separate. The gonad is in a coelomic sac and not directly continuous with the gonoduct. The ovum undergoes incom- plete segmentation. Development is direct. In habit, Cephalopods are predominantly active and predatory ; in diet, carnivorous. The shells of the Nautilus are common on the shores of Far East seas, but the animals are less familiar. The Nautilus swims quickly after crustaceans along the bottom at 30-70 fathoms, and its appearance on the surface, *' floating like a tortoiseshell cat," is probably the result of injury. THE PEARLY NAUTILUS 483 It is called " pearly " on account of the appearance of the innermost layer of the shell. This is exposed after the soft organic stratum and the median porcellanous layer which bears bands of colour have been worn away, or dissolved in a dolphin's stomach, or artificially treated with acid. The beautiful shell is a spiral in one plane, divided into a set of chambers, in the last of which the animal lives, while the others contain gas. The young creature inhabits a tiny shell curved like a horn ; it grows too big for this, and proceeds to enlarge its dwelling, meanwhile drawing itself forward from the older part, and forming a door of lime behind it. This process is repeated again and again ; as an addition is made in front, the animal draws itself forward a little, and shuts off a part of the chamber in which it has been living. All the compart- ments are in communication by a median tube of skin — the siphuncle — which is in part calcareous. It has been suggested that " each septum shutting off an air - containing chamber is formed during a period of quiescence, probably after the reproductive act, when the visceral mass of the Nautilus may be slightly shrunk, and gas is secreted from the dorsal integument so as to fill up the space previously occupied by the animal." There can be no confusion between the beautiful shell of the cuttlefish called the Paper Nautilus {Argonaiita argo) and that of our type. For only the female Argonaut bears a shell ; it is not chambered, and is a shelter for the eggs — a cradle, more than a house. It seems to be formed by two of the arms. It is instructive to compare the Nautilus shell with that of some Gasteropods, for there also chambers are occasion- ally formed. But these arise from secondary alterations of Fig. 274. — Section of shell of Naxitilus. — After T.endenfeld. 484 PHYLUM MOLLUSCA an originally continuous spiral. The Gasteropod shell is usually unsymmetrical, and the foot {ventral) is turned towards the internal curve of the coil, while in Nautilus Fig. 275. — The Pearly Nautilus {Nautilus pompilius). — After Owen. The shell is represented in section, but the animal is not dissected. Part of the mantle has been removed, c. Last or body chamber, separated by a septum (se.) from the compartment behind ; s., the siphuncle traversing all the compartments ; m., the portion of the mantle which is reflected over the shell ; h., the hood ; e., the eye with its opening to the exterior ; /., the lobes which bear the sheathed tentacles {t.) ; si., the incomplete siphon ; mu., the shell muscle ; n., the position of the nidamental gland. the dorsal part of the animal is towards the internal surface of the chamber. There are only some three or four living species of Nautilus, but there are many hundred fossils of this and allied genera. This list is usually swelled by the addition of the extinct Ammonites, but there are some reasons for CLASSIFICATION OF CEPHALOPODS 485 suspecting that these are rather " convergent " than nearly related. The following table states the chief points of distinction between Nautilus and the other series of Cephalopods : — Cephalopoda Tetrabranchiata (Nautilus). All extinct except one genus — Nautilus ; • the extinct forms are usually ranked as Nautiloid and Ammonoid. Shell externa], chambered, straight or bent or spirally coiled. That in which Nautilus lives has been described, with its siphuncle, gas-containing compartments, etc. The part of the foot surrounding the mouth bears a large number of lobes, which carry tentacles in little sheaths, but no suckers. The two mid-lobes of the foot form a siphon, but they are not fused into a tube. The eye is without a lens, and is bathed internally by sea-water, which enters by a small pinhole aperture. There are two " osphradia " or smelUng patches at the bases of the gills. Two pairs of gills ; two pairs of ne- phridia ; two genital ducts (the left rudimentary). The coelom sac (pericardium) opens directly to the exterior by two aper- tures. The heart has two pairs of auricles, and there are no branchial hearts. No ink-bag. No salivary glands. DiBRANCHiATA (Septa, Octopus, etc.). Numerous living genera, ranked as Decapods or Octopods ; along with the former the extinct Belemnites are included. No living Dibranchiate lives in a shell. The shell was internal even in the extinct Belemnites, and in modern forms it occurs in various degrees of degeneration (cf. Spirula, Sepia, Loligo), or is quite absent (Octopoda). The part of the foot surrounding the mouth is divided into ten or eight arms, which carry suckers, stalked in Decapods, sessile in Octopods. The two mid-lobes of the foot fuse to form a completely closed tubular siphon or funnel. The covering of the eye may be per- forated, but the mouth of the retinal cup is closed by a lens. There are no osphradia, though there may be " olfactory pits " behind the eyes. One pair of gills ; one pair of nephridial sacs ; two oviducts in Octopoda and Oigopsida ; two vasa deferentia in Eledone moschata ; in others an un- paired genital duct. The ccelom opens into the nephridia by two pores, and thus to the ex- terior. The heart has two auricles, and there are branchial hearts. Usually with an ink-bag. Salivary glands. Classification of Cephalopoda Order L Tetrabranchiata (see Table). Family I. Nautilida^. Nautilus alone alive ; but a great series of fossil forms, e.g. Ofthoceras, Trochoceras. Family XL Ammonitidce. All extinct, but with shells well preserved, so that long series can be studied. They furnish striking evidence of progressive evolution in definite directions, e.g. Bactrites, Ceratites, Baculites, Ttirrilites, Heteroceras, and the whole series of genera formerly classed as Ammonites. 486 PHYLUM MOLLUSCA Order II. Dibranchiata (see Table). vSub-Order Decapoda. Eight shorter and two longer arms. Suckers stalked and strengthened by a chitinoid ring. Large eyes with a horizontal lid. Body elongated, with lateral fins. Mantle margin with a cartilaginous " hook- and-eye " arrangement. Some sort of internal " shell," enclosed b}"- upgrowths of the mantle. With calcareous internal "shell." Spirula emnites ; Sepia. extinct Bel- FiG. 276. — Section of the shelJ of Spirula. SEPT., Calcareous partition separating chamber from chamber, but perforated by the tubular siphuncle (SIPH.). The initial chamber of the shell, known as the protoconch (PROT.), con- tains a protosiphon (PROS.), but this does not lead into the siphuncle. With organic internal " shell." (a) Eyes with closed cornea, Myopsida, e.g. Loligo. (6) Eyes with open cornea, Oigopsida, e.g. Ommastrephes. Sub-Order Octopoda. Eight arms only. Suckers sessile without chitinoid ring. Small eyes with sphincter-like lid. Body short and rounded. No " hook-and-eye " arrangement. No " shell," except in the female A rgonauta. e.g. Octopus {Polypus), Kledone {Moschites),Argonauta, Cirroteuthis (with cirri on the arms and no radula). SHELLS OF CEPHALOPODS 487 The Nautiloids began in the Cambrian and died out at the end of the Palaeozoic period, except the Orthoceras and Nautilus-\i\i.& types. The genus Nautilus appeared in the Cretaceous. The Ammonite series lasted from the Devonian to the close of the Cretaceous. The two series show a frequent parallelism in archi- tecture. The Cephalopods are the most speciahsed of the Molluscs, and present much diversity of type. They swim freely in the sea, or creep sluggishly among the rocks. They are voracious eaters, and devour very diverse kinds of animals, their parrot-like jaws and powerful odontophore, as well as the numerous suckers, rendering them formidable adversaries. Many live at considerable depths, and their chief foes are the toothed whales, some of which, Uke the sperm whale (Physeter) and the bottle-nose (Hyperoodon), subsist almost entirely on cuttles. Some deep-sea forms have highly evolved luminous organs. Shells of Cephalopods. — A cham- bered external shell, serving as a house, is present in Nautilus alone among living Cephalopods. Most of the extinct forms had large and efficient shells of very- diverse shape, some straight like Ortho- ceras, or coiled, with chambers separated Fig. 277. — Spirula, a small by complex septa, as in the Ammonites. Decapod cuttlefish. In the majority of shells of the Am- jhe mantle (M.) has been partly monitid series, the septa between the chambers are convex towards the aperture (the opposite in the Nautilus) ; the siphuncle is marginal or ventral ; the septal necks of the siphuncle project forwards (not backwards as in the Nautilus) ; there is an initial chamber or protoconch at the apex of the spiral (per- haps represented by a cicatrix in the Nautilus) ; the suture lines cut away to show the spiral chambered shell (SH. ),which has become internal. L., A lumin- escent organ ; F. a terminal fin. 488 PHYLUM MOLLUSCA marking the chambers tend to be lobed. There is often a single or paired " Aptychus," perhaps of the nature of an operculum. Most of the modern forms seem to be more active than their ancestors, and their shells have degenerated. But the line of degeneration is still debated. In Nautilus, although the animal lives within the shell, the mantle fold is for some distance reflected over it ; in the other series of Cephalopods this process has gone farther, and, where a shell is present, it is enclosed within the mantle fold, and is much reduced in size. In the extinct Belemnites the internal shell was straight and chambered, but almost concealed by secondary deposits of lime, secreted by the walls of the shell-sac, and forming the " guard " or rostrum. The conical chambered shell, with a siphuncle, is known as the phragmacone. It is produced anteriorly into a gladius or pro- ostracum. In the extinct Spirulirosira the shell was spiral and mostly internal ; it had a guard. In Spirula the shell can be caught siglit of in the young animal, but it becomes surrounded by the secondary mantle folds that form the mantle-sac It is a spiral chambered shell, with a ventral siphon. Its relation to the dorsal and ventral surface of the animal is the opposite of that of the Nautilus. The shell is inside the animal ; in Natitilus the animal is inside the shell. It seems that Spirula is a swift swimmer at great depths ; though the empty shells are often cast ashore, the creature itself is rarely seen. In Sepia, the narrowed tip of the " bone " probably represents the remains of the phragmacone ; the bulk of the " bone " probably corresponds to the pro-ostracum in the Belemnites. Besides lime there is chitin in the " Sepia-bone." In Loligo there is no deposit of lime, an organic chitinous pen only being left. In Octopus there is no trace of shell at all, and no mantle-pocket, save a trace, in the very yoimg animal. CHAPTER XVII PHYLUM CHORDATA SUB-PHYLUM HEMICHORDA Under the title Hemichorda are included a number of interesting types which seem to have affinities with Verte- brates. These affinities are clearest in certain worm-like animals with distinct gill-clefts, e.g. Balanoglossus and Ptychodera, w^hich form the class Enteropneusta. Perhaps allied to these are two peculiar types — Rhahdopleura and Cephalodiscus, which may be united in the class Ptero- branchia. Very doubtfully allied is Phoronis (see Fig. 153). It will be useful here to take a general survey of the Chordate phylum. First, there may be grouped together as tentative and primitive Chordates — the transitional Hemichorda or Enteropneusts, the mostly degenerate Urochorda or Tuni- cates, and the pioneering Cephalochorda or Lancelets. Second, there are the peculiar and primitive jawless and limbless Cyclostomes or Round Mouths. Third, there is the very successful class of Fishes, including the Dipnoi which have lungs as well as gills. Fourth, there is the dwindling class of Amphibians. Fifth, at a much higher level, with foetal membranes and no branchial respiration, is the heterogeneous class of Reptiles. Sixth and seventh come Birds and Mammals. General Characters of Enteropneusta The worm-like body has three regions — a pre-oral '"pro- boscis,'' a " collar " around and behind the mouth, and a trunk, the anterior part of which bears gill-slits. A dorsal and in part tubular nerve-cord arises from the ectoderm along 489 490 HEMICHORDA the middle line, and is connected, by a ring round the pharynx^ with a ventral cord. In the skin, which is covered with ciliated ectoderm, there is also a nerve plexus. From the anterior region of the gut a diverticulum grows forward for a short distance, becomes a firm support for the proboscis, and is often called the " notochord." The gill-slits open dorsally, are very numerous, and increase in number during life. The mesoderm arises by the outgrowth of five coelom pouches from the archenteron. An unpaired anterior pouch forms the pre-oral or proboscis cavity of the adult ; there are two collar cavities and two trunk cavities. There are about 30 species in 9 genera, e.g. Balanoglossus, Dolichoglossus , Ptychodera, Schizocardium, and Glandiceps . They are very widely, though locally, distributed, and most occur in the littoral area. Description of Balanoglossus Form and habitat. — The species which form this genus are worm-like marine animals, burrowing in sand and mud in almost all seas. They vary in length from about i in. to over 6 in., and are brightly coloured and have a peculiar odour, like that of iodoform. The sexes are distinct, and are marked externally by slight differences in colour. The body consists of a prominent turgid and muscular " pro- boscis," a firm " collar," a region with gill-slits, and, finally, a long, soft, slightly coiled portion. Skin and muscles. — The epidermis is ciliated, and exudes abundant mucus from unicellular glands. With the addition of grains of sand, the mucus sometimes forms a tube round the body. Some species are phosphorescent. The muscular system is best developed about the proboscis and collar, which are used in leisurely locomotion through the soft sand. There are external circular and internal radial and longitudinal muscles. The fibres are unstriped. There is great regenerative capacity. Nervous system. — -The dorsal nerve-cord is most de- veloped in the collar, but is continued along the whole length. It arises as a longitudinal groove of ectoderm and often remains tubular, like a typical spinal cord. The dorsal nerve-cord is connected by a band round the collar BALANOGLOSSUS 491 with a ventral nerve. There is also a nervous plexus beneath the epidermis. There are no special sense organs in the adult. In the larvae of some species there are two eye-spots. ' Alimentary system. — The permanently open mouth is on the ventral surface between the proboscis and the collar. Sand seems to pass into it during the wriggling movements of the animal, which are greatly aided by the turgidity of the proboscis and collar. The pharynx is divided into a dorsal and ventral region, of which the former is respiratory (Fig. 280, g].), and connected with the exterior by many gill- slits, while the latter is nutritive (Fig. 280,^.), and conveys the food particles onwards. Be- hind the region with gill-slits, the gut has a dorsal and a ventral ciliated groove, and bears, throughout the anterior part of its course, numerous glandular sacculations, which can be detected through the skin. The anus is terminal. The animal eats its way through the sand, and derives its food from the nutritive particles and small organisms therein contained. Skeletal system. — The skeletal system is represented by the " notochord," which lies in the^proboscis, and arises, hke the notochord of indubitable Vertebrates, as a diverti- culum from the dorsal wall of the gut in the collar region. Beneath the notochord there is a chitinous " proboscis skeleton." The septa between the gill-slits are supported by chitinous " forked primary " bars ; and each slit, at first circular, is split into a V-shape by the growth downwards of a double rod of chitin called a ''' tongue bar " ; the whole is suggestive of Amphioxus. Fig. 278. — Male of Balanoglossus (Do- lichoglossus) kowalevskii. — After Bate- son. Note anterior proboscis. Mo., Mouth ; op., slight operculum behind the collar ; then the region with gill-slits ; is., testes ; a., anus. 492 HEMICHORDA The body cavity .-The body cavity consists of five Fig. 270.— Dissection of Balanoglossus. r,n r- ^^^Kr,cfic ravitv " Nck., notochord ; N., nerve'; PH., outer Pharynge^^ ^^ ; G.S., gill slits. The main blood vessels are stippled (B.V.). distinct parts, all of which are lined by mesoder,^ and arise as pouches from the archenteron. (a) There is tirst BALANOGLOSSUS 493 the unpaired cavity of the proboscis, which communicates with the exterior by a dorsal pore at the base of the pro- boscis next the collar, (b) In the collar region there are two small paired coelomic cavities, from which two funnels open to the exterior. Both these cavities and that of the proboscis tend to be obliterated by growth of connective tissue, (c) Two other cavities extend along the posterior region of the body, to some extent separated by the dorsal and ventral mesentery which moors the intestine. In these there is a body cavity fluid with cells. /? Fig. 280. — Transverse section through gill-sht region of Ptychodera minuta. — After Spengel. The section, somewhat diagrammatic, shows a gUl-slit (g.s.) to left, and a septum between two slits to the right ; d.n., dorsal nerve ; d.v., dorsal vessel ; v.n., ventral nerve ; v.v., ventral vessel ; g., nutritive part of gut ; gi., respiratory part of gut ; c, lateral coelomic spaces ; l.m., longitudinal muscles ; R., reproductive organs. As the gill-slits are oblique, the whole of one could not be seen in a single cross-section. Respiratory and vascular systems. — The respiratory system consists of many pairs of ciliated gill-slits. They open dorsally by minute pores behind the collar. In development they begin as a pair, increase in number from in front backwards, and they go on increasing long after the adult structure has been attained. Water passes in by the mouth and out by the gill-slits, where it washes branches of the dorsal blood vessel. There is a main dorsal blood vessel, which, at its anterior Fig. 281. — Direct development of Dolichoglossus. — After Bateson. The mesoderm is represented by the broken dark line. I. Section of blastula. II. Beginning of gastrulation ; End., endodcrm. III. Section of gastrula; W., blastopore; ^c.,archenteron; S.c, segmentation cavity, IV., V. Closure of blastopore, outgrowth of five coelom pouches (M.). VI. Longitudinal section, showing the five parts of the body cavity {b.cA, b.c.-, b.c.^) or coelom. VII. Cross-section; C.A^.S., central nervous system ; Nch., notochord ; 6.C.2, body cavity in collar region. VIII, Section at a later stage; D.b.v., dorsal blood vessel. BALANOGLOSSUS 495 end, forms a heart lying above the notochord, and below a closed contractile dilatation, sometimes called the " peri- cardium." Beside the latter there is a paired " proboscis gland," formed from the coelomic epithelium. There is a ventral vessel beneath the gut ; and numerous smaller vessels. The almost colourless blood flows forwards dorsally, backwards ventrally. This system should be contrasted with that of Amphioxus. Excretory and reproductive systems. — No nephridia are known, but from the region of the collar two ciliated funnels open to the exterior, and the enigmatical proboscis gland is possibly excretory. The sexes are separate. A number of simple paired genital organs lie dorsally in a series on each side of the body cavity in and behind the region with gill-slits (Fig. 280, R.). They open by minute dorsal pores. Development. — The eggs are fertilised outside of the body. Segmentation is complete and approximately equal ; a blastula results ; this is invaginated in the normal fashion, and becomes a gastrula. The development may be direct without a larval stage, as in Balanoglossus {Dolichoglossus) kowalevskii, or indirect with a Tornaria larva, as in Balanoglossus himiniensis . In the direct development the blastopore of the gastrula narrows and closes ; the external surface of the gastrula becomes ciliated ; the endoderm lies as an independent closed sac within the ectoderm. Meanwhile the embryo has become or is becoming free from the thin egg envelope, and begins to move about at the bottom in shallow water. It elongates and becomes more worm-like ; there is an anterior tuft and a posterior ring of cilia ; the primitive gut forms five coelomic pouches ; a mouth and an anus are perforated ; there seem to be no fore-gut nor hind-gut invaginations. Two gill-slits appear ; the regions of the body are defined at a very early stage. In the indirect development there is a Tornaria larva, at first bell- shaped. A ventral mouth opens into the curved gut, which is furnished with a posterior terminal anus. A " dorsal pore " leads into a thin- walJed sac which becomes the proboscis cavity of the adult. A pair of coelomic cavities develop from the gut wall, followed by a second pair. Gill pouches arise from the sides of the fore-gut. There are characteristic external ciliated bands, something like those of an Echinoderm larva, for which the Tornaria was originally taken, till Metchnikoff recognised its true nature in 1870. There is also an apical sensory plate (like that of many Annelid trochospheres) with two eye-spots. The Tornaria is a pelagic form. After a period of free pelagic life 496 HEMICHORDA it becomes smaller and more opaque, the ciliated bands become less marked, it sinks to the bottom and metamorphosis sets in. It acquires a proboscis ; the collar becomes marked off, and on its dorsal side a groove closes to form the beginning of the nerve cord. Tongue bars are formed, and the gill pouches open by degrees to the exterior, making gill-slits. The " notochord " grows out from in front. There is an elongation of the post -oral region, and the body begins to lengthen to the worm-like adult form. The Tornaria must be regarded as the more primitive larval state ; the temporary absence of mouth and anus in the direct development is probably an adaptation acquired after the pelagic habit was lost. Johannes iVIuller ranked the Tornaria larva, whose adult form was not then known, beside the larvae of Echinoderms. The ciliated bands of the Tornaria resemble those of Echinoderm larvse, but this is only a superficial character- istic. The anterior pouch, which forms the cavity of the proboscis and communicates with the ex- terior, has also been compared with the beginning of the water-vascular system in Echinoderms, and it is true that in both several independ- ent coelom pouches grow out from Fig. 282.-Tornaria larva, from ^^^^ Primitive gut The anterior the side.-After Spengel. ^^^^ ^f ^^^ ^^ Balano gloss us com- municates with the exterior by a m., Mouth ; g. gut ; a., anus; h., heart ; poj-e, which becomes the proboscis- p., pore entering proboscis cavity; ^ , ,, , ,, , ^, . , c.r., anal ringof cUia ; s.c.r., secondary Pore of the adult, and this has been anail ring. The dark wavy line in- compared with the water-pore, or dicates the margm of the lobes of the outlet of the water-vascular system larval body wath their bands of cilia. ^r x?^\.- i ™ i,- -u • ■ Note also "the apical spot with cilia ^f Echinoderms. which opens simi- and sense organ (fl.s.). larly from an anterior enteroccel to the exterior. Affinities with Vertebrates (especially emphasised by Bateson). (i) ''Notochord." — A dorsal outgrowth from the anterior region of the gut grows forward for a short distance into the pro- boscis, and becomes a solid supporting rod (Fig. 281, Nch.). It may be compared with the notochord of V^ertebrates, which also arises dorsally from the gut. But it lies below the main dorsal blood vessel, is of very limited extent, and may be merely an analogue of the notochord. (2) " Gill-slits." — Numerous gill-slits (Fig. 278) open from the anterior region of the gut to the exterior, and are separated from one another by skeletal bars, which in some ways resemble the framework of the respiratory jiharynx in s.cv- BALANOGLOSSUS 497 (3) Ampkioxus. There are, however, many differences in detail — thus the shts open dorsally, not later- ally ; the skeletal bars arc differ- ently disposed ; the blood supply is different. " Dorsal nerve-cord." — A dorsal median insinking (Fig. 280, d.n.) of ectoderm, especially strong in the region of the collar, ma}' be compared with the medullary canal of Vertebrates. But it must be b.s p.s (4.) Fig. 283. — Piece of a colony of Cephalodiscus, showing the tubes inhabited by the animals. — After Ridewood. noticed that there is also a ventral nerve-cord (Fig. 280, v.n.). ''The coslom.'' — The develop- ment of five enterocoelic pouches is vevy suggestive of affinities with Amphioxus. b-^ Fig. 284. — An individual Cephalo discus. — After Ridewood. h., Buds ; st., stolon ; go., to the left, bulging of the body caused by the gonad ; ga., to the right, bulging of the body caused by the stomach ; p.s., pos- terior lobe of buccal shield ; r.l., a red line on the buccal shield ; b.s., dark edge of the buccal shield ; /'/..tentacular plumes. 32 Affinities with Annelids (after Spengel). The larva (Tornaria) (Fig. 282) may be regarded as a modified Trochosphere, but this points at most to a far-off common stock. Moreover, the neph- ridia,^ usually present in the Trochosphere, are unrepre- sented in the Tornaria. The heart lies, as in some An- nelids, dorsal to the gut, not ventral as in Vertebrates ; the dorsal vessel carries blood for- wards, the ventral backwards, as is usual in Annelids. But the double nervous system is essentially different from 498 HEMICHORDA that t)f Annelids ; and the gill-slits are unrepresented there, though Salensky has described oesophageal pockets opening to the exterior in four Annelid types — Polygordius, Saccocirrus, Spio /iiliginosiis, and Polydora cornuta. In the last there are five pairs in the larva, and two persist. If there be a relationship between Enteropneusta and Annelids, it must be a very distant one, perhaps restricted to origin from some common stock. Class Pterobranchia. (i) Cephalodiscus Cephalodiscus dodecalophus was dredged by the Challenger in the Magellan Straits. Others are known from Japan, the Malay Archi- pelago, South Africa, and the Antarctic. It was at first described by M'Intosh as a divergent Polyzoon, but the researches of Harmer point to relationship with Balanoglossits. The minute individuals are associated together within a gelatinous investment ; the colony may attain a size of 9 in. by 6 in. The gut is curved, the anus being beside the mouth, beneath which are 4-6 pairs of arms with ciliated tentacles. These two characters, formerly supposed to indicate Polyzoan affinities, may perhaps be adaptations to the sedentary life. With Balanoglossus this type has been compared, on account of the possession of the following characters : — (a) The body is divided into three regions, which correspond to the proboscis, collar, and trunk of Balanoglossus ; this is especially obvious in the young bud ; {h) each of the three regions contains a coelomic cavity, the most anterior being single, while the other two are divided by a median partition ; (c) the anterior pre-oral cavity opens to the exterior by two pores (cf. proboscis pore of Balanoglossus) ; (d) the collar region is also furnished with two collar-pores ; {e) in the collar region the dorsal nervous system is also placed, and is continued to some extent into the proboscis ; (/) beneath the nervous system lies a diverti- culum from the gut, which extends towards the proboscis region ; this has been compared to the " notochord " of Balanoglossus ; (g) the anterior region of the gut is perforated by a pair of lateral gill-slits. The gonads lie between anus and pharynx. Buds are given off from a lateral stalk. (2) Rhabdopleura This genus is found at considerable depths in the North Sea and Atlantic. Like Cephalodiscus, the individuals are minute and stalked, and occur in a colony ; in this case, however, they remain attached to one another by a common stolon, instead of being united only by an investment. The proboscis or buccal shield makes a thin annulated tube within which the polyp moves up and down. In the head region there are two hollow lateral arms bearing numerous ciliated tentacles, which have a skeletal support. The gut, as in Cephalodiscus, has a U-shaped curvature and an anterior diverticulum (" notochord "). There are five coelomic cavities, and two collar-pores. There are no gill-slits. CHAPTER XVIII PHYLUM CHORDATA SUB-PHYLUM UROCHORDA or TUNICATA (AsciDiANS, Sea-Squirts, etc.) The Tunicates are remarkable animals, which seem to stumble on the border line between Invertebrates and Vertebrates. They were classified with Polyzoa and Brachiopoda as Molluscoidea, until, in 1866, Kowalevsky described the development of a simple Ascidian, and correlated it, step by step, with that of Amphioxus. He showed that the larval Ascidian has a dorsal nerve-cord, a notochord in the tail region, gill-slits opening from the pharynx to the exterior, and an eye developing from the brain. It is true that in most cases the promise of youth is unfulfilled ; the active larva settles down to a sedentary life, loses tail and notochord, nerve-cord and eye, and becomes strangely deformed. Nevertheless we must now class Tunicates along with the Chordates. Of their possible relations to simpler forms nothing definite is known. General Characters The Tunicates are marine Chordata, but the chordate characteristics — dorsal tubular nervous system, notochord, gill-slits, and brain eye — are in most cases discernible only in the free-swimming larval stages. They usually degenerate in the course of their development, and the adults, which are in most cases sedentary, tend to diverge very widely from the Vertebrate type. Thus the nervous system is generally re- duced to a single gatiglion placed above the pharynx. The 499 500 UROCHORDA body is invested by a thickeiied cuticular test, which contains cellulose. The relatively large pharynx is perforated by two {in Larvacea), or {in the majority) by numerous ciliated gill- slits, and is surrounded to a greater or less extent by a peri- branchial chamber, which communicates with the exterior by a special dorsal {atrial) opening. The ventral heart is simple and tubidar, and there is a periodic reversal in the direction of the blood current. Nephridia are absent, and the renal organs have no ducts. All are hermaphrodite. There is usually a metamorphosis iti development. Colonies are frequently formed. Type of TuNiCATA — a simple Ascidian {Ascidia mentula) An adult Ascidia is an irregular oval of 3 to 4 in. in length ; one end is attached to stones or weed ; the other, more tapering, bears the 8-lobed mouth ; close beside this, on the morphologically dorsal surface, lies the 6-lobed ex- halant or atrial aperture. During life, water is constantly being drawn in by the mouth and passed out by the atrial opening. If irritated, the animal may drive a jet of water with considerable force from both apertures, whence the name " sea-squirt." Test. — The whole body is clothed in a thick test, some- times called a tunic, though this name is more frequently applied to the underlying body wall. From this body wall the test can be readily removed, the two being unattached except at one spot, where blood vessels pass into the test, and also to a less degree at the two openings. To begin with, this test is a true cuticle, produced by secretory prolongations of the ectoderm cells ; but soon after its formation mesenchyme cells migrate into it, and give rise to patches of connective tissue cells. These cells apparently retain throughout life some phagocytic importance. In Ascidia outgrowths of the body wall with prolongations of blood channels enter the test, ramifying in all directions. In some Ascidians this is carried further, so that the test becomes an important accessory organ of respiration. The test consists in great part of a carbohydrate identical with the cellulose of plants. This " cellulose " or " tunicin " is common throughout the group, but the relative amount STRUCTURE OF ASCIDIAN produced varies markedly in the different forms, forms the " test "-cells make calcareous spicules. In. up. In some — En. Fig. 285. — Dissection of Ascidian. — After Herdniaii. In. ap.. Inhalant aperture ; T., test, cut away below to show mus- cular layer, pharynx, etc. ; En., endostyle or ventral groove of pharynx. Note removal of pharynjf to show, on the other — the left — side, stomach {St.), intestine (with fold seen at inci- sion), and reproductive organs (G.) ; H., opening of pharynx into oesophagus; G.D., genital duct; A., anus; CI., cloacal chamber; Ex. ap., exhalant aperture; Gn., lies above the ganglion, which is seen between the two apertures ; beneath it is the sub-neural gland and its duct. Body wall and muscular system. — The body wall, mantle, or tunic, disclosed by peeling off the test, is a struc- ture of considerable complexity. Its outer surface is 502 UROCHORDA covered by a single layer of ectoderm cells, which secrete the test. Beneath these there lies a gelatinous matrix con- taining numerous connective tissue cells, blood-carrying spaces, muscle cells forming slender fibres, and so on. A true coelom has been described in some embryos, but it is afterwards almost suppressed, being represented at most by the pericardium and small lacunar spaces. The apparent body cavity of the Ascidian — the space between gut and body wall — is, as we shall see, lined throughout by ectoderm. The muscular system is not w^ell developed. The muscle cells are much elongated and unstriped ; they are aggre- gated into fibres of varying thickness, which form an ir- regular network on the right side of the body, while they are virtually absent on the left. Special sets of fibres form sphincters round the apertures. Alimentary and respiratory systems. — The mouth opens into a short stomodasum, separated from the branchial sac itself by a sphincter muscle, whose posterior border is furnished with numerous simple elongated tentacles. Behind this lies a ciliated peripharyngeal groove. In the living animal the tentacles form a sort of sieve over the opening of the branchial sac. This sac is morphologically the pharynx, and extends almost to the posterior end of the body. It is separated frorii the mantle by a space whose dimensions vary greatly in the different regions of the body. This space is the peribranchial chamber, which is formed from the ectoderm, and communicates w'ith the exterior by the atrial opening, and with the branchial sac by innumer- able slits. The remainder of the alimentary canal lies on the left side of the body, between pharynx and mantle, and consists of a short oesophagus leading from the pharynx to the fusiform stomach, and of an intestine which describes an S-shaped curve, and then crosses the atrial chamber, to end in an anus lying beneath the exhalant opening. The absorbing surface of the intestine is increased by a marked infolding, corresponding to the typhlosole of the earthworm. A mass of tubules connected by a duct with the cavity of the stomach is possibly a digestive gland. The structure of the pharynx is exceedingly complex, for it has a double function — respiratory and nutritive. More- STRUCTURE OF ASCIDIAN 5O3 over, the breathing organs of sedentary animals tend to be elaborate. The water which enters by the branchial aper- ture is not only used in respiration, but brings with it the minute food particles. Similarly, the outgoing current carries with it the water used in respiration, the undigested residue of the food, and the spermatozoa and ova. The water of respiration passes from the pharynx through its numerous gill openings to the peribranchial chamber, and so to the exterior. On its way it purifies the blood in the vessels running in the complex framework of the pharynx wall. The water-current is produced and maintained by the action of the cihated cells lining the gill-slits, and its force necessitates special arrangements to prevent the food particles being swept out before they have entered the digestive region of the gut. In this connection there is a longitudinal glandular groove or endostyle along the ventral surface of the pharynx, and a ciliated fold on its dorsal — the regions being defined by the nerve ganglion. According to Willey, the minute algae and the like of the food are entangled in the abundant mucus secreted by the ventral groove or endostyle, and are swept forward in a cord of slime, until at the anterior end of the endostyle they reach the ciliated peripharyngeal groove, whose two halves sur- round the pharynx, and unite to form the dorsal lamina or fold. The food particles passing round the peripharyngeal groove are swept backwards by the cilia of the dorsal lamina until they reach the cesophageal opening. In many Ascidians the dorsal lamina is replaced by a series of pro- cesses — the dorsal languets, which may be sensory, as well as food- wafting structures. Nervous system and sense organs. — In the adult both of these show marked degeneration. In the larva there is a slightly developed brain continued into a dorsal nerve-cord, and having connected with it a median eye and an otocyst. The two latter are completely absent in the adult, and the nervous system consists merely of a ganglionic mass lying between the two apertures, giving off a few nerves forwards and backwards. A structure of doubtful utility, but of considerable morphological interest, is the small sub-neural gland which lies beneath the ganglion, and communicates by a ciliated duct with the pharynx. The opening 504 UROCHORDA is usually complex, aud forms the so-called dorsal tubercle, which is very distinct on the wall of the pharynx, and of considerable systematic importance. It lies at the point where the two halves of the ciliated groove, or peripharyngeal band, already described, converge Pk.^ Fig. 286. —Diagram of Ascidian. — After Herdman. The arrows indicate tlie two openings ; the dark border the test. Ph., Pharynx, with gill-slits ; G., reproductive organs ; H., heart, with blood vessels ; G.D., genital ducts ; /?., rectum, ending in cloacal chamber. Surrounding the pharynx the peribranchial cavity is shown. dorsally to form the dorsal lamina. In Ascidia the sub-neural organ is ventral to the brain, and partly glandular in character, and so it is in many ; in some cases, however, it is dorsal in position, and its glandular portion is reduced to nil. It is probable that the sub- neural gland and its duct correspond to the olfactory pit of Amphioxus, and perhaps to the hypophysis of Vertebrates. STRUCTURE OF ASCIDIAN 505 It is probable that the pigment spots between the lobes of the apertures, the tentacles in the branchial siphon, and the dorsal lamina, or its representatives, the languets, have some sensory function. Vascular system. — The simple tubular heart lies in a pericardial space at the ventral side of the lower end of the pharynx. In development, two diverticula grow out from the pharynx ; these meet and fuse, forming the peri- cardium. The heart arises as an invagination from its dorsal wall, and is thus endodermal in origin, and probably not homologous with the heart of the other Vertebrates. A periodical reversal of the direction of the waves of con- traction is discernible in the heart ; for a certain number of beats the blood is driven upwards, and then the direction is reversed. This same reversal also occurs in Phoronis. According to Herdman, the ventro -dorsal contractions occasion the following circulation : — The blood, which is spread out on the walls of the pharynx in vessels lying between the slits, collects into one large (branchio-cardiac) vessel, which, after receiving a vessel from the test, enters the ventral end of the heart. From the dorsal end it is poured into a great (cardio-visceral) trunk, which sends one branch to the test, and then breaks up among the viscera. From the visceral lacunae the blood is collected (in a branchio -visceral trunk) and distributed to the branchial sac. At the reversal of the contractions this circulation is also reversed. The reversal occurs every couple of minutes or so. The blood is very colourless, but usually contains a few pigmented corpuscles. Excretory system. — In the loop of the intestine there lies a mass of clear vesicles containing uric acid and other waste products. This, therefore, seems to be a renal organ, but there is no duct. Bacteria are usually found in the vesicles, and their activity may make diffusion easier. It is interesting to find such a plant-like method of storing up, instead of eliminating, waste products in these very passive animals. It has been suggested that the sub-neural gland may have some renal function. Reproductive system. — Tunicates are hermaphrodite. The reproductive organs (Fig. 285, G.) are very simple, and lie in the loop of the intestine. The ovary is the larger, and contains a cavity into which the ova are set free, and from which they pass outwards along an oviduct which opens into the cloacal chamber. The testis surrounds the ovary, and is mature at a different time (dichogamy) ; its duct runs by the side of the oviduct. In some forms, 5o6 UROCHORDA where the gonads are near the cloaca, there are no ducts. The ova are surrounded by foUicular cells, and probably fertilised in the cloaca. The development shows three steps : — (i) from the fertlHsed ovum to the free -swimming larval stage ; (2) the larval or so-called " tad- pole " stage ; (3) iixation and degeneration into the adult Ascidian. The spherical blast ula forms a gastrula by invagination. Over- growth of the gastrular lip results in the covering over of the dorsal nerve rudiment and the formation of an ectodermic neiural canal, open in front at the neuropore and for a time communicating behind with the archenteron. The embryo elongates. Internally the endo- NP E NCH Fig. 287. — Young embryo of Ascidian [Clavelina). — After Van Beneden and Julin. NP., Neuropore ; N.C., neural canal ; NCH., notochord ; E., ectoderm; M., mesoderm ; Jl., archenteron. derm of the archenteron gives rise to the notochord. Mesoderm cells spread between the archenteron and outer ectoderm and form the side muscles of the growing tail. The neuropore closes and the neural canal becomes a spinal cord, ending in front in a closed brain vesicle where a dorsal unpaired eye and balancing organ develop (Fig. 288). In the free-swimming larva a mouth opens into the pharynx in front of the brain vesicle. On either side an ectodermal invagination appears — these later coalesce dorsally and form the peribranchial chamber of the adult — and fuses with outgrowths from the pharynx where the first two pairs of gill-shts break through. The tadpole- like larva, with dorsal tubular nervous system, notochord in the tail DEVELOPMENT OF ASCIDIAN 507 region, and pharyngeal gill-slits, swims for hours by its tail, and then fixes, head downwards, by three glandular papillae. Degeneration begins. Two main processes go on concurrently : — (i) the disappearance of the tail, and with it the notochord and most of the nervous system ; (2) the growth of test, pharynx, and gonads. The tail is consumed by phagox:ytes. The peribranchial chamber becomes greatly enlarged, opening at the atrial aperture. The pharynx becomes large, and the gill-slits increase in number. Exceptional upward growth of the originally short region between fixing papilla? and mouth results in the adult relation of parts. The whole meta- morphosis is one of the most signal instances of degeneration. en. Fig. 288. — Embryo of Clavelina. — Modified after Seehger. f.p., Fixing papilla ; e.f., ectodermic fold ; e.g., ciliated groove ; en., endostyle ; s.o., cerebral vesicle with sense organs ; g.s., gill-slits ; n., nerve-cord beginning to degenerate ; c/?., noto- chord ; g., gut curving upwards towards atrial opening. The atrial invagination is marked by a dotted line ; the mouth and atrial opening are indicated by arrows. General Notes on Tunicata The description of Ascidia given above is, in its general outlines, applicable to all the simple Ascidians, which are abundantly represented on British coasts. As contrasted with this type, we have in other members of the class most remarkable diversity in structure, habit, and life-history. The simple Ascidians are usually sedentary, growing fixed to stones, shells, or weed, and are widely distributed, occurring on or near the coasts of all seas. With the exception of the so-called social Ascidians {e.g. Clavelina), they do not reproduce by budding, but are often gregarious, great masses being found together. To the compound Ascidians {e.g. Botryllus) the simple forms are linked by Clavelina, where each individual is 5o8 UROCHORDA surrounded by its own test, but is united to its fellows by a common blood system. In the compound Ascidians, on the other hand, many individuals are enveloped in a common test, and all, like Clavelina, possess the power of reproducing asexually by budding. There is, however, no doubt that the so-called compound Ascidians are an artificial group, whose members diverge widely in structure, though all display the two characters mentioned. Some of the compound Ascidians are not fixed, but form floating colonies. These forms lead up to the beautiful Fig. 2iS9. — Part of a colony of Boiryllus, showing two individuals embedded in a gelatinous matrix and with a common exhalant aperture. C.E.A., Coiumon exhalant aperture ; I. A., inhalant aperture ; B.S., blindly ending blood sinuses of colony ; PH., pharynx ; H., heart ; £., endostyle ; D.L., dorsal lamina ; 6'., stomach ; A'^., nerve ganglion ; PB., peribranchial space. Pyrosoma or phosphorescent fire-flame, where the whole colony with its numerous individuals swims as one creature. All these belong to the Ascidian series, and display interesting diversity in their methods of development. The simplest case is that already described for Asa'dia, where the tailed larva gives rise to a sexual adult without any power of budding. This occurs in almost all simple Ascidians, but even here there are indications of possible complication. Thus, on the one hand, in some, e.g. Mol- gula, there is a tendency towards abbreviation — the larval stage being suppressed; while, on the other, the adult LIFE-HISTORY OF TUNICATES 509 acquires the power of reproducing asexually, e.g. Clavelina. Both processes are carried further in the compound Ascidians. In these the eggs have usually a considerable amount of yolk, and development takes place either in the atrial cavity of the mother, or in special brood-pouches. In consequence, the development, especially in the early stages, shows considerable modification, although the larval stage is quite distinct. Again, the tailed larva develops into an adult which has no sexual organs, but forms a colony by budding. The individuals of the colony then give rise to eggs and so to larvae. The development thus includes a distinct alternation of generations. Fig. 290. — Asexual reproduction in. Salpa. At the Kinder end of the adult endostyle a stolon develops containing tubular out- growths. The stolon breaks out through the test of the adult and becomes constricted to form a series of buds. ST., Stolon growing out from ventral surface of parent salp (P.) ; 5., S' ., S"., buds at different stages of development. Budding takes place in many different ways in the com- pound Ascidians. In one set (the Diplosomidae) the tailed larva is precociously reproductive, giving rise to buds before undergoing metamorphosis. This, forms an interesting transition to the condition seen in Pyrosoma, where the fertilised egg gives rise to a rudimentary larva (cyatho- zooid), from which a young colony of four individuals arises by budding. These individuals again bud, until a large colony is formed, the members of which become sexual. The ova are few in number, a statement which is generally true for the pelagic Tunicates, as contrasted with sedentary forms. 510 UROCHORDA While the Ascidians in the narrow sense indude all the more typical Tunicates, there are two other sets, few in Fig. 291. — " Nurse " of Doliolum mulleri. — After Uljanin. /., Inhalant, £., exhalant aperture ; C, ciliated band round pharynx (P.) ; En., endostyle ; 0., " otocyst " ; A''., nerve- ganglion ; H., heart ; CE., assophageal opening ; D., stomach ; A., anus ; CI., cloaca ; D.O., dorsal organ ; M., muscle bands. Fig. 292. — Sexual individual of Doliolum mulleri. — After Uljanin. G., gonads ; B., gill-slits ; other letters as before. The unlettered reference line points to the stomach. number both as regards genera and species, but of great theoretic importance. LIFE-HISTORY OF SALPS 5" The one set includes the free-swimming genera Salpa and Doliolmn, together with the aberrant deep-water genus Octacnemus ; the other, a few active free-swimming forms, which exhibit throughout Hfe many of the characteristics of the larval Ascidian. Of these, Appendicular ia is the most familiar type. Both Salpa and Doliolum are pelagic in habit, and differ markedly in structure from the Ascidians. The body is fusiform (Salpa) or barrel- shaped {Doliolum), and wholly or partially encircled by definite muscle bands, which replace the scattered fibres of the Ascidians. The mouth is at one end of the body, and the atrial aperture at the other ; the animals swim bv forcing the water out of the peribranchial chamber a.a PPb ph s.ns Fig. 293. — Diagram of Salpa africana. o.a., Oral aperture ; d.t., dorsal tubercle ; te., tentacle ; g., ganglion ; m., muscle bands ; atr., atrium ; b.v., blood vessel ; an., anus ; a.a., exhalant aperture ; v.n., visceral nucleus ; h., heart ; St., stolon ; d.L, dorsal lamina ; E., endostyle ; s.n.g., sub-neural gland ; ph., pharynx ; p.p.b., peri-pharyngeal band. posteriorly. Many of the most marked signs of specialisation in the Ascidians are here absent. Thus the test may be, as in Doliolum, very thin and devoid of cells, and the branchial sac is relatively simple in structure ; the ciha on its walls are never so important in producing the respiratory current as in the Ascidians, and the gill-slits may be few in number, or, as in Salpa, may be represented by two large holes in the walls of the pharynx. Further, the hermaphroditism is modified by the occurrence of very marked protogyny, and the ova are never numerous — in Salpa each sexual individual usually produces only one. On the other hand, the development exhibits marked alternation of generations, both solitary and colonial forms being included in one life- history. In Doliolum the fertilised egg gives rise to a tailed larva, which develops into an asexual " nurse," possessing the power of budding (of. 512 UROCHORDA Compound Ascidians). The ventral stolon of the nurse gives rise to a number of primitive buds, which migrate over the body until they reach a dorsal outgrowth, apparently well supplied with blood. Here they fix themselves and di\'ide up to form three series of buds — two lateral and one median. All these buds develop into individuals belonging to the sexual generation, but only a few become truly sexual. The two lateral series develop into nutritive forms, which supply the nurse with food. The nurse itself loses its alimentary and respiratory organs, and PP 7^ 7?b' Fig. 294. — Structure of Appendtcularla. Herdman. -After S.O., Sense organ; br., branchial aperture ; at. (upper side), dorsal tubercle; ot., otocyst ; n.g., nerve ganglion; pp., peri- pharyngeal band ; n., nerve-cord ; ce., oesophagus; St., stomach ; ov., ovary ; tes., testes ; 1., intestine ; /;., heart ; tt., urochord, cut at u'. ; n.g'., n.g"., nerve gangliapf taU ; m., muscle band ; ' app., tail cut through ; a., anus ; at., one of the atrial aper- tures ; end., endostyle. becomes a mere organ of locomotion. The median buds develop into " foster mothers," which ultimately go free, bearing with them other buds destined to develop into the solitary sexual forms. In these, first ova and then spermatozoa are produced, which start the life-cycle afresh. It is thus obvious that there is considerable division of labour in the sexual form, accompanied by polymorphism ; the whole process presents some curious analogies to the conditions seen in the Coelentera. In Salpa the single egg is fertilised within the body of the mother, and becomes attached to the wall of the peribranchial chamber. Here the developing egg is nourished by means of a " placenta," and the ORDERS OF TUNICATES 513 development is in consequence much abbreviated, the tailed larva not being represented. This embryo gives rise to a soUtary " nurse " form, which by budding produces a chain of embryos. This chain is set free ; its members become sexual, and, either while still united or after separation, give rise to the eggs which develop into the nurse form. The remaining order of Tunicates includes minute simplified forms like Appendicularia, also pelagic in habitat, but without any. power of budding, and never forming colonies. These forms have a distinct tail, which is bent at an angle to the body, and is the main organ of locomo- tion. The mouth is at the anterior end ; the anus, which is distinct from the atrial openings, is at the root of the tail. These atrial openings lie slightly behind the anus, and are merely small ectodermic invagina- tions communicating with the two gill-slits of the pharynx. They correspond to the similar invaginations in the Ascidian larva. The test may form a large investing " house," but it does not contain cells, and is periodically cast and renewed. The important points as regards internal structure are the presence of the notochord throughout hfe, and the structure of the nervous system. The latter consists of a lobed ganglionic mass above the mouth, and a dorsal nerve-cord extending backward from this into the tail, where it is furnished with other gangha. In connection with the cerebral ganglion there is a pigment spot, an otocyst (auditory?), and a tubular process communicating with the pharynx, and corresponding to the sub-neural gland and the cihated duct of other Tunicates. We have already noted the simple structure of the pharynx, which has but two gill-slits communicating directly with the exterior. The same simphcity of structure is observable in the heart, which is without any associated vessels. The hermaphrodite reproductive organs he posteriorly, and open to the exterior by a very fine duct on the dorsal surface. As contrasted with Salpa and DoHolum, the animals are protandrous, and not protogynous. The development is unknown. Classification.— Order i. Larvacea Free-swimming, pelagic, and solitary forms provided with a large locomotor tail containing a notochord. The pharynx opens to the exterior by two ventral cihated slits, and there is no peribranchial chamber. The nervous system extends into the tail region. A relatively large cuticular " house " is formed as a secretion round the animal ; it is periodically cast off and rapidly replaced. The house acts as a most efficient filtering apparatus for capfuring minute diatoms and protozoa upon which the animal feeds. The Larvacea or Appendicu- larians are of special interest because they show little or no degeneration, and retain throughout life the chordate characters which other Tunicates lose during metamorphosis. Appendicularia, Oikopleura, Fritillaria, Megalocercus, Kowalevskia. Order 2. Ascidiacea Ascidians which may be fixed or free, simple or colonial, but which in the adult have no tail and no trace of notochord. There is a large 33 514 UROCHORDA branchial sac opening by many slits into the peribranchial chamber' which communicates with the exterior by a single opening. There is a permanent and well-developed cuticular test into which cells from the body migrate. Many have the power of budding, and there is sometimes alternation of generations. Sub-order i. Ascidiae Simplices. Solitary fixed forms which rarely bud ; when colonial, each individual has a separate test. Ascidia, Phallusia, Ciona. Sub-order 2. Ascidiae Composita?. Fixed Ascidians which repro- duce by gemmation, the individuals being embedded in a common investing mass. Botryllus, Polyclinum. Sub-order 3. Ascidiae Luciae. Free-swimming Ascidians which re- produce by gemmation to form a colony, having the shape of a hollow cylinder, open at one end. There is one genus, Pyrosoma, widely represented, especially in tropical seas. They are brilliantly phosphorescent, and some attain a length of twelve feet. Order 3. Thaliacea Free-swimming pelagic forms, which may be either single or ** social," and in the adult are never provided with tail or notochord. The muscles are in the form of distinct circular bands, which effect locomotion by squirting out the water from the body. The test, which may be well or ill developed, is always transparent. The life- history exhibits distinct alternation of generations, and there is some- times polymorphism. {a) Cyclomyaria. Muscle bands form complete rings. Doliolum, Anchinia. (6) Hemimyaria. Muscle bands are in the form of incomplete rings. Salpa, Octacnemus. Relationships The questions as to the origin of the Tunicates and the relations of the orders are too difficult to be discussed here, but we may note that there are two possible views as to the position of Appendicularia and its allies. They may be regarded as the slightly modified descendants of the primitive Tunicates, from which the Ascidians have diverged in the direction of degeneration, or as prematurely sexual larva? derived from an already degraded Ascidian-like form. Both views have had supporters, and the one adopted materially affects the general method of regarding the group. In any case the Larvacea retain persistently a number of characters which were probably possessed by the primitive Tunicata. There are several resemblances between Tunicates and Lancelets (see the next chapter), e.g. the relatively large respiratory pharynx and the peribranchial cavity, but this probably does not mean more than that both groups arose from a common stock of primitive chordate animals. CHAPTER XIX PHYLUM CHORDATA SUB -PHYLUM CEPHALOCHORDA This small sub-phylum includes about sixteen species, popularly known as lancelets. The type represents an offshoot from the primitive Vertebrate stock, lost, it is to be feared, for ever ; but while some authorities regard it as a pioneer-type and as a far-off prophecy of a fish, others hold it to be degenerate— a " weed in the Vertebrate garden." It is possible that both views are right, and that the lancelet is a somewhat degenerate pioneer. General Characters There is a dorsal tubular nerve-cord, but no well-defined brain region. The notochord is persistent and unsegmented ; it is surrounded by a continuous sheath, and projects in a unique manner in front of the anterior end of the nerve-cord. In the adult the gill-slits are very numerous, and open into an atrial or peribranchial cavity. The body wall is built up of over fifty myotomes. From Fishes, the lancelets are widely removed by the absence of limbs, skull, jaws, differentiated brain, sympathetic nervous system, eye, ear, definite heart, spleen, and genital ducts. There are numerous separate nephridia. The gonads are numerous and arranged seg- mentally. The larval form is strangely asymmetrical and the larval period is prolonged. The species have a wide distribution, like many old-fashioned animals. They occur near the coasts in warm and temperate seas, are sluggish in habit, and feed on microscopic organisms or organic particles. 515 5i6 CEPHALOCHORDA Amphioxus lanceolatus, the best-known species Mode of life. — The lancelets are fond of lying in the sand in water about two fathoms deep, with only the fringed aperture of the mouth projecting. They feed on diatoms and other small organisms, which are sucked into the mouth. At times, especially in the evening, the adults start up and swim about, but they are never so active as the larvae. The early embryo is pelagic. It is of interest to note that along with lancelets, specimens of the Annelid Ophelia are often obtained ; they closely resemble lancelets,